CSS Syntax Module Level 3
CSS Syntax Module Level 3
W3C Candidate Recommendation Draft
24 December 2021
More details about this document
This version:
Latest published version:
Editor's Draft:
Previous Versions:
History:
Implementation Report:
Test Suite:
Feedback:
CSSWG Issues Repository
Editors:
Tab Atkins Jr.
Google
Simon Sapin
Mozilla
Suggest an Edit for this Spec:
GitHub Editor
W3C
MIT
ERCIM
Keio
Beihang
). W3C
liability
trademark
and
permissive document license
rules apply.
Abstract
This module describes, in general terms, the basic structure and syntax of CSS stylesheets. It defines, in detail, the syntax and parsing of CSS - how to turn a stream of bytes into a meaningful stylesheet.
CSS
is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, etc.
Status of this document
This section describes the status of this document at the time of its publication.
A list of current W3C publications
and the latest revision of this technical report
can be found in the
W3C technical reports index at https://www.w3.org/TR/.
This document was published
by the
CSS Working Group
as a
Candidate Recommendation Draft
using the
Recommendation
track
Publication as a Candidate Recommendation
does not imply endorsement by
W3C
and its Members.
A Candidate Recommendation Draft
integrates changes from the previous Candidate Recommendation
that the Working Group intends to include in a subsequent Candidate Recommendation Snapshot.
This is a draft document
and may be updated, replaced
or obsoleted by other documents at any time.
It is inappropriate to cite this document as other than work in progress.
Please send feedback
by
filing issues in GitHub
(preferred),
including the spec code “css-syntax” in the title, like this:
“[css-syntax]
…summary of comment…
”.
All issues and comments are
archived
Alternately, feedback can be sent to the (
archived
) public mailing list
www-style@w3.org
This document is governed by the
2 November 2021 W3C Process Document
This document was produced by a group operating under the
W3C Patent Policy
W3C maintains a
public list of any patent disclosures
made in connection with the deliverables of the group;
that page also includes instructions for disclosing a patent.
An individual who has actual knowledge of a patent which the individual believes
contains
Essential Claim(s)
must disclose the information in accordance with
section 6 of the W3C Patent Policy
1.
Introduction
This section is not normative.
This module defines the abstract syntax and parsing of CSS stylesheets
and other things which use CSS syntax
(such as the HTML
style
attribute).
It defines algorithms for converting a stream of Unicode
code points
(in other words, text)
into a stream of CSS tokens,
and then further into CSS objects
such as stylesheets, rules, and declarations.
1.1.
Module interactions
This module defines the syntax and parsing of CSS stylesheets.
It supersedes the lexical scanner and grammar defined in CSS 2.1.
2.
Description of CSS’s Syntax
This section is not normative.
A CSS document is a series of
style rules
which are
qualified rules
that apply styles to elements in a document—
and
at-rules
which define special processing rules or values for the CSS document.
qualified rule
starts with a prelude
then has a {}-wrapped block containing a sequence of declarations.
The meaning of the prelude varies based on the context that the rule appears in—
for
style rules
, it’s a selector which specifies what elements the declarations will apply to.
Each declaration has a name,
followed by a colon and the declaration value.
Declarations are separated by semicolons.
A typical rule might look something like this:
p > a {
color: blue;
text-decoration: underline;
In the above rule, "
p > a
" is the selector,
which, if the source document is HTML,
selects any
elements that are children of a
element.
color: blue
" is a declaration specifying that,
for the elements that match the selector,
their
color
property should have the value
blue
Similarly, their
text-decoration
property should have the value
underline
At-rules
are all different, but they have a basic structure in common.
They start with an "@"
code point
followed by their name as a CSS keyword.
Some
at-rules
are simple statements,
with their name followed by more CSS values to specify their behavior,
and finally ended by a semicolon.
Others are blocks;
they can have CSS values following their name,
but they end with a {}-wrapped block,
similar to a
qualified rule
Even the contents of these blocks are specific to the given
at-rule
sometimes they contain a sequence of declarations, like a
qualified rule
other times, they may contain additional blocks, or at-rules, or other structures altogether.
Here are several examples of
at-rules
that illustrate the varied syntax they may contain.
@import "my-styles.css";
The
@import
at-rule
is a simple statement.
After its name, it takes a single string or
url()
function to indicate the stylesheet that it should import.
@page :left {
margin-left: 4cm;
margin-right: 3cm;
The
@page
at-rule
consists of an optional page selector (the
:left
pseudoclass),
followed by a block of properties that apply to the page when printed.
In this way, it’s very similar to a normal style rule,
except that its properties don’t apply to any "element",
but rather the page itself.
@media print {
body { font-size: 10pt }
The
@media
at-rule
begins with a media type
and a list of optional media queries.
Its block contains entire rules,
which are only applied when the
@media
s conditions are fulfilled.
Property names and
at-rule
names are always
ident sequences
which have to start with an
ident-start code point
, two hyphens,
or a hyphen followed by an ident-start code point,
and then can contain zero or more
ident code points
You can include any
code point
at all,
even ones that CSS uses in its syntax,
by
escaping
it.
The syntax of selectors is defined in the
Selectors spec
Similarly, the syntax of the wide variety of CSS values is defined in the
Values & Units spec
The special syntaxes of individual
at-rules
can be found in the specs that define them.
2.1.
Escaping
This section is not normative.
Any Unicode
code point
can be included in an
ident sequence
or quoted string
by
escaping
it.
CSS escape sequences start with a backslash (\), and continue with:
Any Unicode
code point
that is not a
hex digits
or a
newline
The escape sequence is replaced by that
code point
Or one to six
hex digits
, followed by an optional
whitespace
The escape sequence is replaced by the Unicode
code point
whose value is given by the hexadecimal digits.
This optional whitespace allow hexadecimal escape sequences
to be followed by "real" hex digits.
An
ident sequence
with the value "&B"
could be written as
\26 B
or
\000026B
A "real" space after the escape sequence must be doubled.
2.2.
Error Handling
This section is not normative.
When errors occur in CSS,
the parser attempts to recover gracefully,
throwing away only the minimum amount of content
before returning to parsing as normal.
This is because errors aren’t always mistakes—
new syntax looks like an error to an old parser,
and it’s useful to be able to add new syntax to the language
without worrying about stylesheets that include it being completely broken in older UAs.
The precise error-recovery behavior is detailed in the parser itself,
but it’s simple enough that a short description is fairly accurate.
At the "top level" of a stylesheet,
an
starts an at-rule.
Anything else starts a qualified rule,
and is included in the rule’s prelude.
This may produce an invalid selector,
but that’s not the concern of the CSS parser—
at worst, it means the selector will match nothing.
Once an at-rule starts,
nothing is invalid from the parser’s standpoint;
it’s all part of the at-rule’s prelude.
Encountering a
ends the at-rule immediately,
while encountering an opening curly-brace
<{-token>
starts the at-rule’s body.
The at-rule seeks forward, matching blocks (content surrounded by (), {}, or [])
until it finds a closing curly-brace
<}-token>
that isn’t matched by anything else
or inside of another block.
The contents of the at-rule are then interpreted according to the at-rule’s own grammar.
Qualified rules work similarly,
except that semicolons don’t end them;
instead, they are just taken in as part of the rule’s prelude.
When the first {} block is found,
the contents are always interpreted as a list of declarations.
When interpreting a list of declarations,
unknown syntax at any point causes the parser to throw away whatever declaration it’s currently building,
and seek forward until it finds a semicolon (or the end of the block).
It then starts fresh, trying to parse a declaration again.
If the stylesheet ends while any rule, declaration, function, string, etc. are still open,
everything is automatically closed.
This doesn’t make them invalid,
though they may be incomplete
and thus thrown away when they are verified against their grammar.
After each construct (declaration, style rule, at-rule) is parsed,
the user agent checks it against its expected grammar.
If it does not match the grammar,
it’s
invalid
and gets
ignored
by the UA,
which treats it as if it wasn’t there at all.
3.
Tokenizing and Parsing CSS
User agents must use the parsing rules described in this specification
to generate the
[CSSOM]
trees from text/css resources.
Together, these rules define what is referred to as the CSS parser.
This specification defines the parsing rules for CSS documents,
whether they are syntactically correct or not.
Certain points in the parsing algorithm are said to be
parse errors
The error handling for parse errors is well-defined:
user agents must either act as described below when encountering such problems,
or must abort processing at the first error that they encounter for which they do not wish to apply the rules described below.
Conformance checkers must report at least one parse error condition to the user
if one or more parse error conditions exist in the document
and must not report parse error conditions
if none exist in the document.
Conformance checkers may report more than one parse error condition if more than one parse error condition exists in the document.
Conformance checkers are not required to recover from parse errors,
but if they do,
they must recover in the same way as user agents.
3.1.
Overview of the Parsing Model
The input to the CSS parsing process consists of a stream of Unicode
code points
which is passed through a tokenization stage followed by a tree construction stage.
The output is a CSSStyleSheet object.
Note:
Implementations that do not support scripting do not have to actually create a CSSOM CSSStyleSheet object,
but the CSSOM tree in such cases is still used as the model for the rest of the specification.
3.2.
The input byte stream
When parsing a stylesheet,
the stream of Unicode
code points
that comprises the input to the tokenization stage
might be initially seen by the user agent as a stream of bytes
(typically coming over the network or from the local file system).
If so, the user agent must decode these bytes into
code points
according to a particular character encoding.
To
decode
stylesheet
’s stream of bytes into a stream of
code points
Determine the fallback encoding
of
stylesheet
and let
fallback
be the result.
Decode
stylesheet
’s stream of bytes
with fallback encoding
fallback
and return the result.
Note:
The
decode
algorithm
gives precedence to a byte order mark (BOM),
and only uses the fallback when none is found.
To
determine the fallback encoding
of a
stylesheet
If HTTP or equivalent protocol provides an
encoding label
(e.g. via the charset parameter of the Content-Type header) for the
stylesheet
get an encoding
from
encoding label
If that does not return failure,
return it.
Otherwise, check
stylesheet
’s byte stream.
If the first 1024 bytes of the stream begin with the hex sequence
40 63 68 61 72 73 65 74 20 22 XX* 22 3B
where each
XX
byte is a value between 0
16
and 21
16
inclusive
or a value between 23
16
and 7F
16
inclusive,
then
get an encoding
from a string formed out of
the sequence of
XX
bytes,
interpreted as
ASCII
What does that byte sequence mean?
The byte sequence above,
when decoded as ASCII,
is the string "
@charset "…";
",
where the "…" is the sequence of bytes corresponding to the encoding’s label.
If the return value was
utf-16be
or
utf-16le
return
utf-8
if it was anything else except failure,
return it.
Why use utf-8 when the declaration says utf-16?
The bytes of the encoding declaration spell out “
@charset "…";
” in ASCII,
but UTF-16 is not ASCII-compatible.
Either you’ve typed in complete gibberish (like
䁣桡牳整•utf-16be∻
) to get the right bytes in the document,
which we don’t want to encourage,
or your document is actually in an ASCII-compatible encoding
and your encoding declaration is lying.
Either way, defaulting to UTF-8 is a decent answer.
As well, this mimics the behavior of HTML’s
attribute.
Note:
Note that the syntax of an encoding declaration
looks like
the syntax of an
at-rule
named
@charset
but no such rule actually exists,
and the rules for how you can write it are much more restrictive than they would normally be for recognizing such a rule.
A number of things you can do in CSS that would produce a valid
@charset
rule (if one existed),
such as using multiple spaces, comments, or single quotes,
will cause the encoding declaration to not be recognized.
This behavior keeps the encoding declaration as simple as possible,
and thus maximizes the likelihood of it being implemented correctly.
Otherwise, if an
environment encoding
is provided by the referring document,
return it.
Otherwise, return
utf-8
Though UTF-8 is the default encoding for the web,
and many newer web-based file formats assume or require UTF-8 encoding,
CSS was created before it was clear which encoding would win,
and thus can’t automatically assume the stylesheet is UTF-8.
Stylesheet authors
should
author their stylesheets in UTF-8,
and ensure that either an HTTP header (or equivalent method) declares the encoding of the stylesheet to be UTF-8,
or that the referring document declares its encoding to be UTF-8.
(In HTML, this is done by adding a
element to the head of the document.)
If neither of these options are available,
authors should begin the stylesheet with a UTF-8 BOM
or the exact characters
@charset "utf-8";
Document languages that refer to CSS stylesheets that are decoded from bytes
may define an
environment encoding
for each such stylesheet,
which is used as a fallback when other encoding hints are not available or can not be used.
The concept of
environment encoding
only exists for compatibility with legacy content.
New formats and new linking mechanisms
should not
provide an
environment encoding
so the stylesheet defaults to UTF-8 instead in the absence of more explicit information.
Note:
[HTML]
defines
the environment encoding for
Note:
[CSSOM]
defines
the environment encoding for
Note:
[CSS-CASCADE-3]
defines
the environment encoding for
@import
3.3.
Preprocessing the input stream
The
input stream
consists of the
filtered code points
pushed into it as the input byte stream is decoded.
To
filter code points
from a stream of (unfiltered)
code points
input
Replace any U+000D CARRIAGE RETURN (CR)
code points
U+000C FORM FEED (FF)
code points
or pairs of U+000D CARRIAGE RETURN (CR) followed by U+000A LINE FEED (LF) in
input
by a single U+000A LINE FEED (LF)
code point
Replace any U+0000 NULL or
surrogate
code points
in
input
with U+FFFD REPLACEMENT CHARACTER (�).
4.
Tokenization
To
tokenize
a stream of
code points
into a stream of CSS tokens
input
repeatedly
consume a token
from
input
until an
is reached,
pushing each of the returned tokens into a stream.
Note:
Each call to the
consume a token
algorithm
returns a single token,
so it can also be used "on-demand" to tokenize a stream of
code points
during
parsing,
if so desired.
The output of tokenization step is a stream of zero or more of the following tokens:
<[-token>
<]-token>
<(-token>
<)-token>
<{-token>
and
<}-token>
, and
have a value composed of zero or more
code points
Additionally, hash tokens have a type flag set to either "id" or "unrestricted". The type flag defaults to "unrestricted" if not otherwise set.
has a value composed of a single
code point
, and
have a numeric value.
and
additionally have a type flag set to either "integer" or "number". The type flag defaults to "integer" if not otherwise set.
additionally have a unit composed of one or more
code points
Note:
The type flag of hash tokens is used in the Selectors syntax
[SELECT]
Only hash tokens with the "id" type are valid
ID selectors
4.1.
Token Railroad Diagrams
This section is non-normative.
This section presents an informative view of the tokenizer,
in the form of railroad diagrams.
Railroad diagrams are more compact than an explicit parser,
but often easier to read than an regular expression.
These diagrams are
informative
and
incomplete
they describe the grammar of "correct" tokens,
but do not describe error-handling at all.
They are provided solely to make it easier to get an intuitive grasp of the syntax of each token.
Diagrams with names such as
represent tokens.
The rest are productions referred to by other diagrams.
comment
newline
whitespace
hex digit
escape
ws*
4.2.
Definitions
This section defines several terms used during the tokenization phase.
next input code point
The first
code point
in the
input stream
that has not yet been consumed.
current input code point
The last
code point
to have been consumed.
reconsume the current input code point
Push the
current input code point
back onto the front of the
input stream
so that the next time you are instructed to consume the
next input code point
it will instead reconsume the
current input code point
EOF code point
A conceptual
code point
representing the end of the
input stream
Whenever the
input stream
is empty,
the
next input code point
is always an EOF code point.
digit
code point
between U+0030 DIGIT ZERO (0) and U+0039 DIGIT NINE (9) inclusive.
hex digit
digit
or a
code point
between U+0041 LATIN CAPITAL LETTER A (A) and U+0046 LATIN CAPITAL LETTER F (F) inclusive,
or a
code point
between U+0061 LATIN SMALL LETTER A (a) and U+0066 LATIN SMALL LETTER F (f) inclusive.
uppercase letter
code point
between U+0041 LATIN CAPITAL LETTER A (A) and U+005A LATIN CAPITAL LETTER Z (Z) inclusive.
lowercase letter
code point
between U+0061 LATIN SMALL LETTER A (a) and U+007A LATIN SMALL LETTER Z (z) inclusive.
letter
An
uppercase letter
or a
lowercase letter
non-ASCII code point
code point
with a value equal to or greater than U+0080
ident-start code point
letter
non-ASCII code point
or U+005F LOW LINE (_).
ident code point
An
ident-start code point
digit
or U+002D HYPHEN-MINUS (-).
non-printable code point
code point
between U+0000 NULL and U+0008 BACKSPACE inclusive,
or U+000B LINE TABULATION,
or a
code point
between U+000E SHIFT OUT and U+001F INFORMATION SEPARATOR ONE inclusive,
or U+007F DELETE.
newline
U+000A LINE FEED.
Note that U+000D CARRIAGE RETURN and U+000C FORM FEED are not included in this definition,
as they are converted to U+000A LINE FEED during
preprocessing
whitespace
newline
, U+0009 CHARACTER TABULATION, or U+0020 SPACE.
maximum allowed code point
The greatest
code point
defined by Unicode: U+10FFFF.
ident sequence
A sequence of
code points
that has the same syntax as an
Note:
The part of an
after the "@",
the part of a
(with the "id" type flag) after the "#",
the part of a
before the "(",
and the unit of a
are all
ident sequences
representation
The
representation
of a token
is the subsequence of the
input stream
consumed by the invocation of the
consume a token
algorithm
that produced it.
This is preserved for a few algorithms that rely on subtle details of the input text,
which a simple "re-serialization" of the tokens might disturb.
The
representation
is only consumed by internal algorithms,
and never directly exposed,
so it’s not actually required to preserve the exact text;
equivalent methods,
such as associating each token with offsets into the source text,
also suffice.
Note:
In particular, the
representation
preserves details
such as whether .009 was written as
.009
or
9e-3
and whether a character was written literally
or as a CSS escape.
The former is necessary to properly parse
productions;
the latter is basically an accidental leak of the tokenizing abstraction,
but allowed because it makes the impl easier to define.
If a token is ever produced by an algorithm directly,
rather than thru the tokenization algorithm in this specification,
its representation is the empty string.
4.3.
Tokenizer Algorithms
The algorithms defined in this section transform a stream of
code points
into a stream of tokens.
4.3.1.
Consume a token
This section describes how to
consume a token
from a stream of
code points
It will return a single token of any type.
Consume comments
Consume the
next input code point
whitespace
Consume as much
whitespace
as possible.
Return a
U+0022 QUOTATION MARK (")
Consume a string token
and return it.
U+0023 NUMBER SIGN (#)
If the
next input code point
is an
ident code point
or the
next two input code points
are a valid escape
then:
Create a
If the
next 3 input code points
would start an ident sequence
set the
’s type flag to "id".
Consume an ident sequence
and set the
’s value to the returned string.
Return the
Otherwise,
return a
with its value set to the
current input code point
U+0027 APOSTROPHE (')
Consume a string token
and return it.
U+0028 LEFT PARENTHESIS (()
Return a
<(-token>
U+0029 RIGHT PARENTHESIS ())
Return a
<)-token>
U+002B PLUS SIGN (+)
If the input stream
starts with a number
reconsume the current input code point
consume a numeric token
and return it.
Otherwise,
return a
with its value set to the
current input code point
U+002C COMMA (,)
Return a
U+002D HYPHEN-MINUS (-)
If the input stream
starts with a number
reconsume the current input code point
consume a numeric token
and return it.
Otherwise,
if the
next 2 input code points
are
U+002D HYPHEN-MINUS
U+003E GREATER-THAN SIGN
(->),
consume them
and return a
Otherwise,
if the input stream
starts with an ident sequence
reconsume the current input code point
consume an ident-like token
and return it.
Otherwise,
return a
with its value set to the
current input code point
U+002E FULL STOP (.)
If the input stream
starts with a number
reconsume the current input code point
consume a numeric token
and return it.
Otherwise,
return a
with its value set to the
current input code point
U+003A COLON (:)
Return a
U+003B SEMICOLON (;)
Return a
U+003C LESS-THAN SIGN (<)
If the
next 3 input code points
are
U+0021 EXCLAMATION MARK
U+002D HYPHEN-MINUS
U+002D HYPHEN-MINUS
(!--),
consume them
and return a
Otherwise,
return a
with its value set to the
current input code point
U+0040 COMMERCIAL AT (@)
If the
next 3 input code points
would start an ident sequence
consume an ident sequence
create an
with its value set to the returned value,
and return it.
Otherwise,
return a
with its value set to the
current input code point
U+005B LEFT SQUARE BRACKET ([)
Return a
<[-token>
U+005C REVERSE SOLIDUS (\)
If the input stream
starts with a valid escape
reconsume the current input code point
consume an ident-like token
and return it.
Otherwise,
this is a
parse error
Return a
with its value set to the
current input code point
U+005D RIGHT SQUARE BRACKET (])
Return a
<]-token>
U+007B LEFT CURLY BRACKET ({)
Return a
<{-token>
U+007D RIGHT CURLY BRACKET (})
Return a
<}-token>
digit
Reconsume the current input code point
consume a numeric token
and return it.
ident-start code point
Reconsume the current input code point
consume an ident-like token
and return it.
EOF
Return an
anything else
Return a
with its value set to the
current input code point
4.3.2.
Consume comments
This section describes how to
consume comments
from a stream of
code points
It returns nothing.
If the
next two input code point
are
U+002F SOLIDUS (/) followed by a U+002A ASTERISK (*),
consume them
and all following
code points
up to and including
the first U+002A ASTERISK (*) followed by a U+002F SOLIDUS (/),
or up to an EOF code point.
Return to the start of this step.
If the preceding paragraph ended by consuming an EOF code point,
this is a
parse error
Return nothing.
4.3.3.
Consume a numeric token
This section describes how to
consume a numeric token
from a stream of
code points
It returns either a
, or
Consume a number
and let
number
be the result.
If the
next 3 input code points
would start an ident sequence
then:
Create a
with the same value and type flag as
number
and a unit set initially to the empty string.
Consume an ident sequence
Set the
’s unit to the returned value.
Return the
Otherwise,
if the
next input code point
is U+0025 PERCENTAGE SIGN (%),
consume it.
Create a
with the same value as
number
and return it.
Otherwise,
create a
with the same value and type flag as
number
and return it.
4.3.4.
Consume an ident-like token
This section describes how to
consume an ident-like token
from a stream of
code points
It returns an
, or
Consume an ident sequence
, and let
string
be the result.
If
string
’s value is an
ASCII case-insensitive
match for "url",
and the
next input code point
is U+0028 LEFT PARENTHESIS ((),
consume it.
While the
next two input code points
are
whitespace
consume the
next input code point
If the
next one or two input code points
are U+0022 QUOTATION MARK ("),
U+0027 APOSTROPHE ('),
or
whitespace
followed by U+0022 QUOTATION MARK (") or U+0027 APOSTROPHE ('),
then create a
with its value set to
string
and return it.
Otherwise,
consume a url token
and return it.
Otherwise,
if the
next input code point
is U+0028 LEFT PARENTHESIS ((),
consume it.
Create a
with its value set to
string
and return it.
Otherwise,
create an
with its value set to
string
and return it.
4.3.5.
Consume a string token
This section describes how to
consume a string token
from a stream of
code points
It returns either a
or
This algorithm may be called with an
ending code point
which denotes the
code point
that ends the string.
If an
ending code point
is not specified,
the
current input code point
is used.
Initially create a
with its value set to the empty string.
Repeatedly consume the
next input code point
from the stream:
ending code point
Return the
EOF
This is a
parse error
Return the
newline
This is a
parse error
Reconsume the current input code point
create a
, and return it.
U+005C REVERSE SOLIDUS (\)
If the
next input code point
is EOF,
do nothing.
Otherwise,
if the
next input code point
is a newline,
consume it.
Otherwise,
(the stream
starts with a valid escape
consume an escaped code point
and append the returned
code point
to the
’s value.
anything else
Append the
current input code point
to the
’s value.
4.3.6.
Consume a url token
This section describes how to
consume a url token
from a stream of
code points
It returns either a
or a
Note:
This algorithm assumes that the initial "url(" has already been consumed.
This algorithm also assumes that it’s being called to consume an "unquoted" value,
like
url(foo)
A quoted value, like
url("foo")
is parsed as a
Consume an ident-like token
automatically handles this distinction;
this algorithm shouldn’t be called directly otherwise.
Initially create a
with its value set to the empty string.
Consume as much
whitespace
as possible.
Repeatedly consume the
next input code point
from the stream:
U+0029 RIGHT PARENTHESIS ())
Return the
EOF
This is a
parse error
Return the
whitespace
Consume as much
whitespace
as possible.
If the
next input code point
is U+0029 RIGHT PARENTHESIS ()) or EOF,
consume it and return the
(if EOF was encountered, this is a
parse error
);
otherwise,
consume the remnants of a bad url
create a
and return it.
U+0022 QUOTATION MARK (")
U+0027 APOSTROPHE (')
U+0028 LEFT PARENTHESIS (()
non-printable code point
This is a
parse error
Consume the remnants of a bad url
create a
and return it.
U+005C REVERSE SOLIDUS (\)
If the stream
starts with a valid escape
consume an escaped code point
and append the returned
code point
to the
’s value.
Otherwise,
this is a
parse error
Consume the remnants of a bad url
create a
and return it.
anything else
Append the
current input code point
to the
’s value.
4.3.7.
Consume an escaped code point
This section describes how to
consume an escaped code point
It assumes that the U+005C REVERSE SOLIDUS (\) has already been consumed
and that the next input code point has already been verified
to be part of a valid escape.
It will return a
code point
Consume the
next input code point
hex digit
Consume as many
hex digits
as possible, but no more than 5.
Note that this means 1-6 hex digits have been consumed in total.
If the
next input code point
is
whitespace
consume it as well.
Interpret the
hex digits
as a hexadecimal number.
If this number is zero,
or is for a
surrogate
or is greater than the
maximum allowed code point
return U+FFFD REPLACEMENT CHARACTER (�).
Otherwise, return the
code point
with that value.
EOF
This is a
parse error
Return U+FFFD REPLACEMENT CHARACTER (�).
anything else
Return the
current input code point
4.3.8.
Check if two code points are a valid escape
This section describes how to
check if two code points are a valid escape
The algorithm described here can be called explicitly with two
code points
or can be called with the input stream itself.
In the latter case, the two
code points
in question are
the
current input code point
and the
next input code point
in that order.
Note:
This algorithm will not consume any additional
code point
If the first
code point
is not U+005C REVERSE SOLIDUS (\),
return false.
Otherwise,
if the second
code point
is a
newline
return false.
Otherwise, return true.
4.3.9.
Check if three code points would start an ident sequence
This section describes how to
check if three code points would start an
ident sequence
The algorithm described here can be called explicitly with three
code points
or can be called with the input stream itself.
In the latter case, the three
code points
in question are
the
current input code point
and the
next two input code points
in that order.
Note:
This algorithm will not consume any additional
code points
Look at the first
code point
U+002D HYPHEN-MINUS
If the second
code point
is an
ident-start code point
or a U+002D HYPHEN-MINUS,
or the second and third
code points
are a valid escape
return true.
Otherwise, return false.
ident-start code point
Return true.
U+005C REVERSE SOLIDUS (\)
If the first and second
code points
are a valid escape
return true.
Otherwise, return false.
anything else
Return false.
4.3.10.
Check if three code points would start a number
This section describes how to
check if three code points would start a number
The algorithm described here can be called explicitly with three
code points
or can be called with the input stream itself.
In the latter case, the three
code points
in question are
the
current input code point
and the
next two input code points
in that order.
Note:
This algorithm will not consume any additional
code points
Look at the first
code point
U+002B PLUS SIGN (+)
U+002D HYPHEN-MINUS (-)
If the second
code point
is a
digit
return true.
Otherwise,
if the second
code point
is a U+002E FULL STOP (.)
and the third
code point
is a
digit
return true.
Otherwise, return false.
U+002E FULL STOP (.)
If the second
code point
is a
digit
return true.
Otherwise, return false.
digit
Return true.
anything else
Return false.
4.3.11.
Consume an ident sequence
This section describes how to
consume an ident sequence
from a stream of
code points
It returns a string containing
the largest name that can be formed from adjacent
code points
in the stream, starting from the first.
Note:
This algorithm does not do the verification of the first few
code points
that are necessary to ensure the returned
code points
would constitute an
If that is the intended use,
ensure that the stream
starts with an ident sequence
before calling this algorithm.
Let
result
initially be an empty string.
Repeatedly consume the
next input code point
from the stream:
ident code point
Append the
code point
to
result
the stream
starts with a valid escape
Consume an escaped code point
Append the returned
code point
to
result
anything else
Reconsume the current input code point
Return
result
4.3.12.
Consume a number
This section describes how to
consume a number
from a stream of
code points
It returns a numeric
value
and a
type
which is either "integer" or "number".
Note:
This algorithm does not do the verification of the first few
code points
that are necessary to ensure a number can be obtained from the stream.
Ensure that the stream
starts with a number
before calling this algorithm.
Execute the following steps in order:
Initially set
type
to "integer".
Let
repr
be the empty string.
If the
next input code point
is U+002B PLUS SIGN (+) or U+002D HYPHEN-MINUS (-),
consume it and append it to
repr
While the
next input code point
is a
digit
consume it and append it to
repr
If the
next 2 input code points
are
U+002E FULL STOP (.) followed by a
digit
then:
Consume them.
Append them to
repr
Set
type
to "number".
While the
next input code point
is a
digit
, consume it and append it to
repr
If the
next 2 or 3 input code points
are
U+0045 LATIN CAPITAL LETTER E (E) or U+0065 LATIN SMALL LETTER E (e),
optionally followed by U+002D HYPHEN-MINUS (-) or U+002B PLUS SIGN (+),
followed by a
digit
then:
Consume them.
Append them to
repr
Set
type
to "number".
While the
next input code point
is a
digit
, consume it and append it to
repr
Convert
repr
to a number
and set the
value
to the returned value.
Return
value
and
type
4.3.13.
Convert a string to a number
This section describes how to
convert a string to a number
It returns a number.
Note:
This algorithm does not do any verification to ensure that the string contains only a number.
Ensure that the string contains only a valid CSS number
before calling this algorithm.
Divide the string into seven components,
in order from left to right:
sign
a single U+002B PLUS SIGN (+) or U+002D HYPHEN-MINUS (-),
or the empty string.
Let
be the number -1 if the sign is U+002D HYPHEN-MINUS (-);
otherwise, let
be the number 1.
An
integer part
zero or more
digits
If there is at least one digit,
let
be the number formed by interpreting the digits as a base-10 integer;
otherwise, let
be the number 0.
decimal point
a single U+002E FULL STOP (.),
or the empty string.
fractional part
zero or more
digits
If there is at least one digit,
let
be the number formed by interpreting the digits as a base-10 integer
and
be the number of digits;
otherwise, let
and
be the number 0.
An
exponent indicator
a single U+0045 LATIN CAPITAL LETTER E (E) or U+0065 LATIN SMALL LETTER E (e),
or the empty string.
An
exponent sign
a single U+002B PLUS SIGN (+) or U+002D HYPHEN-MINUS (-),
or the empty string.
Let
be the number -1 if the sign is U+002D HYPHEN-MINUS (-);
otherwise, let
be the number 1.
An
exponent
zero or more
digits
If there is at least one digit,
let
be the number formed by interpreting the digits as a base-10 integer;
otherwise, let
be the number 0.
Return the number
s·(i + f·10
-d
)·10
te
4.3.14.
Consume the remnants of a bad url
This section describes how to
consume the remnants of a bad url
from a stream of
code points
"cleaning up" after the tokenizer realizes that it’s in the middle of a
rather than a
It returns nothing;
its sole use is to consume enough of the input stream to reach a recovery point
where normal tokenizing can resume.
Repeatedly consume the
next input code point
from the stream:
U+0029 RIGHT PARENTHESIS ())
EOF
Return.
the input stream
starts with a valid escape
Consume an escaped code point
This allows an escaped right parenthesis ("\)") to be encountered without ending the
This is otherwise identical to the "anything else" clause.
anything else
Do nothing.
5.
Parsing
The input to the parsing stage is a stream or list of tokens from the tokenization stage.
The output depends on how the parser is invoked,
as defined by the entry points listed later in this section.
The parser output can consist of at-rules,
qualified rules,
and/or declarations.
The parser’s output is constructed according to the fundamental syntax of CSS,
without regards for the validity of any specific item.
Implementations may check the validity of items as they are returned by the various parser algorithms
and treat the algorithm as returning nothing if the item was invalid according to the implementation’s own grammar knowledge,
or may construct a full tree as specified
and "clean up" afterwards by removing any invalid items.
The items that can appear in the tree are:
at-rule
An
at-rule
has a name,
a prelude consisting of a list of component values,
and an optional block consisting of a simple {} block.
Note:
This specification places no limits on what an at-rule’s block may contain.
Individual at-rules must define whether they accept a block,
and if so,
how to parse it
(preferably using one of the parser algorithms or entry points defined in this specification).
qualified rule
A qualified rule has
a prelude consisting of a list of component values,
and a block consisting of a simple {} block.
Note:
Most qualified rules will be style rules,
where the prelude is a selector
[SELECT]
and the block a
list of declarations
declaration
Conceptually, declarations are a particular instance of
associating a property or descriptor name with a value.
Syntactically, a declaration has a name,
a value consisting of a list of component values,
and an
important
flag which is initially unset.
Declarations are further categorized
as
property declarations
or
descriptor declarations
with the former setting CSS
properties
and appearing most often in
qualified rules
and the latter setting CSS
descriptors
which appear only in
at-rules
(This categorization does not occur at the Syntax level;
instead, it is a product of where the declaration appears,
and is defined by the respective specifications defining the given rule.)
component value
A component value is one of the
preserved tokens
function
or a
simple block
preserved tokens
Any token produced by the tokenizer
except for
s,
<{-token>
s,
<(-token>
s,
and
<[-token>
s.
Note:
The non-
preserved tokens
listed above are always consumed into higher-level objects,
either functions or simple blocks,
and so never appear in any parser output themselves.
Note:
The tokens
<}-token>
s,
<)-token>
s,
<]-token>
, and
are always parse errors,
but they are preserved in the token stream by this specification to allow other specs,
such as Media Queries,
to define more fine-grained error-handling
than just dropping an entire declaration or block.
function
A function has a name
and a value consisting of a list of component values.
simple block
{}-block
[]-block
()-block
A simple block has an associated token (either a
<[-token>
<(-token>
, or
<{-token>
and a value consisting of a list of component values.
{}-block
[]-block
, and
()-block
refer specifically
to a
simple block
with that corresponding associated token.
5.1.
Parser Railroad Diagrams
This section is non-normative.
This section presents an informative view of the parser,
in the form of railroad diagrams.
These diagrams are
informative
and
incomplete
they describe the grammar of "correct" stylesheets,
but do not describe error-handling at all.
They are provided solely to make it easier to get an intuitive grasp of the syntax.
Stylesheet
Rule list
At-rule
Qualified rule
Declaration list
Declaration
!important
Component value
{} block
() block
[] block
Function block
5.2.
Definitions
current input token
The token or
component value
currently being operated on, from the list of tokens produced by the tokenizer.
next input token
The token or
component value
following the
current input token
in the list of tokens produced by the tokenizer.
If there isn’t a token following the
current input token
the
next input token
is an
A conceptual token representing the end of the list of tokens.
Whenever the list of tokens is empty,
the
next input token
is always an
consume the next input token
Let the
current input token
be the current
next input token
adjusting the
next input token
accordingly.
reconsume the current input token
The next time an algorithm instructs you to
consume the next input token
instead do nothing
(retain the
current input token
unchanged).
5.3.
Parser Entry Points
The algorithms defined in this section produce high-level CSS objects
from lists of CSS tokens.
The algorithms here are operate on a token stream as input,
but for convenience
can also be invoked with a number of other value types.
To
normalize into a token stream
a given
input
If
input
is a list of CSS tokens,
return
input
If
input
is a list of CSS component values,
return
input
Note:
The only difference between a list of tokens
and a list of component values
is that some objects that "contain" things,
like functions or blocks,
are a single entity in the component-value list,
but are multiple entities in a token list.
This makes no difference to any of the algorithms in this specification.
If
input
is a
string
then
filter code points
from
input
tokenize
the result,
and return the final result.
Assert: Only the preceding types should be passed as
input
Note:
Other specs can define additional entry points for their own purposes.
The following notes should probably be translated into normative text in the relevant specs,
hooking this spec’s terms:
Parse a stylesheet
" is intended to be the normal parser entry point,
for parsing stylesheets.
Parse a list of rules
" is intended for the content of at-rules such as
@media
It differs from "
Parse a stylesheet
" in the handling of
and
Parse a rule
" is intended for use by the
CSSStyleSheet#insertRule
method,
and similar functions which might exist,
which parse text into a single rule.
Parse a declaration
" is used in
@supports
conditions.
[CSS3-CONDITIONAL]
Parse a list of declarations
" is for the contents of a
style
attribute,
which parses text into the contents of a single style rule.
Parse a component value
" is for things that need to consume a single value,
like the parsing rules for
attr()
Parse a list of component values
" is for the contents of presentational attributes,
which parse text into a single declaration’s value,
or for parsing a stand-alone selector
[SELECT]
or list of Media Queries
[MEDIAQ]
as in
Selectors API
or the
media
HTML attribute.
5.3.1.
Parse something according to a CSS grammar
It is often desirable to parse a string or token list
to see if it matches some CSS grammar,
and if it does,
to destructure it according to the grammar.
This section provides a generic hook for this kind of operation.
It should be invoked like
"parse
foo
as a CSS
, or similar.
This algorithm returns either failure,
if the input does not match the provided grammar,
or the result of parsing the input according to the grammar,
which is an unspecified structure corresponding to the provided grammar specification.
The return value must only be interacted with by specification prose,
where the representation ambiguity is not problematic.
If it is meant to be exposed outside of spec language,
the spec using the result must explicitly translate it into a well-specified representation,
such as, for example, by invoking a CSS serialization algorithm
(like "serialize as a CSS
value").
Note:
This algorithm,
and
parse a comma-separated list according to a CSS grammar
are
usually
the only parsing algorithms other specs will want to call.
The remaining parsing algorithms are meant mostly for
[CSSOM]
and related "explicitly constructing CSS structures" cases.
Consult the CSSWG for guidance first
if you think you need to use one of the other algorithms.
To
parse something according to a CSS grammar
(aka simply
parse
given an
input
and a CSS
grammar
production:
Normalize
input
and set
input
to the result.
Parse a list of component values
from
input
and let
result
be the return value.
Attempt to match
result
against
grammar
If this is successful,
return the matched result;
otherwise, return failure.
5.3.2.
Parse A Comma-Separated List According To A CSS Grammar
While one can definitely
parse
a value
according to a grammar with commas in it,
if
any
part of the value fails to parse,
the entire thing doesn’t parse,
and returns failure.
Sometimes that’s what’s desired
(such as in list-valued CSS properties);
other times,
it’s better to let each comma-separated sub-part of the value parse separately,
dealing with the parts that parse successfully one way,
and the parts that fail to parse another way
(typically ignoring them,
such as in
).
This algorithm provides an easy hook to accomplish exactly that.
It returns a list of values split by "top-level" commas,
where each values is either failure
(if it failed to parse)
or the result of parsing
(an unspecified structure,
as described in the
parse
algorithm).
To
parse a comma-separated list according to a CSS grammar
(aka
parse a list
given an
input
and a CSS
grammar
production:
Normalize
input
and set
input
to the result.
If
input
contains only
s,
return an empty
list
Parse a comma-separated list of component values
from
input
and let
list
be the return value.
For each
item
of
list
replace
item
with the result of
parsing
item
with
grammar
Return
list
5.3.3.
Parse a stylesheet
To
parse a stylesheet
from an
input
given an optional
url
location
If
input
is a byte stream for stylesheet,
decode bytes
from
input
and set
input
to the result.
Normalize
input
and set
input
to the result.
Create a new stylesheet, with its
location
set to
location
(or null, if
location
was not passed).
Consume a list of rules
from
input
with the
top-level flag
set,
and set the stylesheet’s value to the result.
Return the stylesheet.
5.3.4.
Parse a list of rules
To
parse a list of rules
from
input
Normalize
input
and set
input
to the result.
Consume a list of rules
from the
input
with the
top-level flag
unset.
Return the returned list.
5.3.5.
Parse a rule
To
parse a rule
from
input
Normalize
input
and set
input
to the result.
While the
next input token
from
input
is a
consume the next input token
from
input
If the
next input token
from
input
is an
return a syntax error.
Otherwise,
if the
next input token
from
input
is an
consume an at-rule
from
input
and let
rule
be the return value.
Otherwise,
consume a qualified rule
from
input
and let
rule
be the return value.
If nothing was returned,
return a syntax error.
While the
next input token
from
input
is a
consume the next input token
from
input
If the
next input token
from
input
is an
return
rule
Otherwise, return a syntax error.
5.3.6.
Parse a declaration
Note:
Unlike "
Parse a list of declarations
",
this parses only a declaration and not an at-rule.
To
parse a declaration
from
input
Normalize
input
and set
input
to the result.
While the
next input token
from
input
is a
consume the next input token
If the
next input token
from
input
is not an
return a syntax error.
Consume a declaration
from
input
If anything was returned, return it.
Otherwise, return a syntax error.
5.3.7.
Parse a style block’s contents
Note:
This algorithm parses the contents of
style rules
which need to allow
nested
style rules
and other
at-rules
If you don’t need nested
style rules
such as in
@page
or in
@keyframes
child rules,
use
parse a list of declarations
To
parse a style block’s contents
from
input
Normalize
input
and set
input
to the result.
Consume a style block’s contents
from
input
and return the result.
5.3.8.
Parse a list of declarations
Note:
Despite the name,
this actually parses a mixed list of declarations and at-rules,
as CSS 2.1 does for
@page
Unexpected at-rules (which could be all of them, in a given context)
are invalid and will be ignored by the consumer.
Note:
This algorithm does not handle nested
style rules
If your use requires that,
use
parse a style block’s contents
To
parse a list of declarations
from
input
Normalize
input
and set
input
to the result.
Consume a list of declarations
from
input
and return the result.
5.3.9.
Parse a component value
To
parse a component value
from
input
Normalize
input
and set
input
to the result.
While the
next input token
from
input
is a
consume the next input token
from
input
If the
next input token
from
input
is an
return a syntax error.
Consume a component value
from
input
and let
value
be the return value.
While the
next input token
from
input
is a
consume the next input token
If the
next input token
from
input
is an
return
value
Otherwise,
return a syntax error.
5.3.10.
Parse a list of component values
To
parse a list of component values
from
input
Normalize
input
and set
input
to the result.
Repeatedly
consume a component value
from
input
until an
is returned,
appending the returned values (except the final
) into a list.
Return the list.
5.3.11.
Parse a comma-separated list of component values
To
parse a comma-separated list of component values
from
input
Normalize
input
and set
input
to the result.
Let
list of cvls
be an initially empty list of component value lists.
Repeatedly
consume a component value
from
input
until an
or
is returned,
appending the returned values (except the final
or
) into a list.
Append the list to
list of cvls
If it was a
that was returned,
repeat this step.
Return
list of cvls
5.4.
Parser Algorithms
The following algorithms comprise the parser.
They are called by the parser entry points above.
These algorithms may be called with a list of either tokens or of component values.
(The difference being that some tokens are replaced by
functions
and
simple blocks
in a list of component values.)
Similar to how the input stream returned EOF code points to represent when it was empty during the tokenization stage,
the lists in this stage must return an
when the next token is requested but they are empty.
An algorithm may be invoked with a specific list,
in which case it consumes only that list
(and when that list is exhausted,
it begins returning
s).
Otherwise,
it is implicitly invoked with the same list as the invoking algorithm.
5.4.1.
Consume a list of rules
To
consume a list of rules
given a
top-level flag
Create an initially empty list of rules.
Repeatedly consume the
next input token
Do nothing.
Return the list of rules.
If the
top-level flag
is set,
do nothing.
Otherwise,
reconsume the current input token
Consume a qualified rule
If anything is returned,
append it to the list of rules.
Reconsume the current input token
Consume an at-rule
and append the returned value to the list of rules.
anything else
Reconsume the current input token
Consume a qualified rule
If anything is returned,
append it to the list of rules.
5.4.2.
Consume an at-rule
To
consume an at-rule
Consume the next input token
Create a new at-rule
with its name set to the value of the
current input token
its prelude initially set to an empty
list
and its value initially set to nothing.
Repeatedly consume the
next input token
Return the at-rule.
This is a
parse error
Return the at-rule.
<{-token>
Consume a simple block
and assign it to the at-rule’s block.
Return the at-rule.
simple block
with an associated token of
<{-token>
Assign the block to the at-rule’s block.
Return the at-rule.
anything else
Reconsume the current input token
Consume a component value
Append the returned value to the at-rule’s prelude.
5.4.3.
Consume a qualified rule
To
consume a qualified rule
Create a new qualified rule
with its prelude initially set to an empty
list
and its value initially set to nothing.
Repeatedly consume the
next input token
This is a
parse error
Return nothing.
<{-token>
Consume a simple block
and assign it to the qualified rule’s block.
Return the qualified rule.
simple block
with an associated token of
<{-token>
Assign the block to the qualified rule’s block.
Return the qualified rule.
anything else
Reconsume the current input token
Consume a component value
Append the returned value to the qualified rule’s prelude.
5.4.4.
Consume a style block’s contents
To
consume a style block’s contents
Create an initially empty
list
of declarations
decls
and an initially empty
list
of rules
rules
Repeatedly consume the
next input token
Do nothing.
Extend
decls
with
rules
, then return
decls
Reconsume the current input token
Consume an at-rule
and append the result to
rules
Initialize a temporary list initially filled with the
current input token
As long as the
next input token
is anything other than a
or
consume a component value
and append it to the temporary list.
Consume a declaration
from the temporary list.
If anything was returned,
append it to
decls
with a value of "&" (U+0026 AMPERSAND)
Reconsume the current input token
Consume a qualified rule
If anything was returned,
append it to
rules
anything else
This is a
parse error
Reconsume the current input token
As long as the
next input token
is anything other than a
or
consume a component value
and throw away the returned value.
5.4.5.
Consume a list of declarations
To
consume a list of declarations
Create an initially empty list of declarations.
Repeatedly consume the
next input token
Do nothing.
Return the list of declarations.
Reconsume the current input token
Consume an at-rule
Append the returned rule to the list of declarations.
Initialize a temporary list initially filled with the
current input token
As long as the
next input token
is anything other than a
or
consume a component value
and append it to the temporary list.
Consume a declaration
from the temporary list.
If anything was returned,
append it to the list of declarations.
anything else
This is a
parse error
Reconsume the current input token
As long as the
next input token
is anything other than a
or
consume a component value
and throw away the returned value.
5.4.6.
Consume a declaration
Note:
This algorithm assumes that the
next input token
has already been checked to be an
To
consume a declaration
Consume the next input token
Create a new declaration
with its name set to the value of the
current input token
and its value initially set to an empty
list
While the
next input token
is a
consume the next input token
If the
next input token
is anything other than a
this is a
parse error
Return nothing.
Otherwise,
consume the next input token
While the
next input token
is a
consume the next input token
As long as the
next input token
is anything other than an
consume a component value
and append it to the declaration’s value.
If the last two non-
s in the declaration’s value are
with the value "!"
followed by an
with a value that is an
ASCII case-insensitive
match for "important",
remove them from the declaration’s value
and set the declaration’s
important
flag to true.
While the last token in the declaration’s value is a
remove
that token.
Return the declaration.
5.4.7.
Consume a component value
To
consume a component value
Consume the next input token
If the
current input token
is a
<{-token>
<[-token>
, or
<(-token>
consume a simple block
and return it.
Otherwise, if the
current input token
is a
consume a function
and return it.
Otherwise, return the
current input token
5.4.8.
Consume a simple block
Note:
This algorithm assumes that the
current input token
has already been checked to be an
<{-token>
<[-token>
, or
<(-token>
To
consume a simple block
The
ending token
is the mirror variant of the
current input token
(E.g. if it was called with
<[-token>
, the
ending token
is
<]-token>
.)
Create a
simple block
with its associated token set to the
current input token
and with its value initially set to an empty
list
Repeatedly consume the
next input token
and process it as follows:
ending token
Return the block.
This is a
parse error
Return the block.
anything else
Reconsume the current input token
Consume a component value
and append it to the value of the block.
Note:
CSS has an unfortunate syntactic ambiguity
between blocks that can contain declarations
and blocks that can contain qualified rules,
so any "consume" algorithms that handle rules
will initially use this more generic algorithm
rather than the more specific
consume a list of declarations
or
consume a list of rules
algorithms.
These more specific algorithms are instead invoked when grammars are applied,
depending on whether it contains a
or a
5.4.9.
Consume a function
Note:
This algorithm assumes that the
current input token
has already been checked to be a
To
consume a function
Create a function with its name equal to the value of the
current input token
and with its value initially set to an empty
list
Repeatedly consume the
next input token
and process it as follows:
<)-token>
Return the function.
This is a
parse error
Return the function.
anything else
Reconsume the current input token
Consume a component value
and append the returned value
to the function’s value.
6.
The
An+B
microsyntax
Several things in CSS,
such as the
:nth-child()
pseudoclass,
need to indicate indexes in a list.
The
An+B
microsyntax is useful for this,
allowing an author to easily indicate single elements
or all elements at regularly-spaced intervals in a list.
The
An+B
notation defines an integer step (
) and offset (
),
and represents the
An+B
th elements in a list,
for every positive integer or zero value of
with the first element in the list having index 1 (not 0).
For values of
and
greater than 0,
this effectively divides the list into groups of
elements
(the last group taking the remainder),
and selecting the
th element of each group.
The
An+B
notation also accepts the
even
and
odd
keywords,
which have the same meaning as
2n
and
2n+1
, respectively.
Examples:
2n+0 /* represents all of the even elements in the list */
even /* same */
4n+1 /* represents the 1st, 5th, 9th, 13th, etc. elements in the list */
The values of
and
can be negative,
but only the positive results of
An+B
for
≥ 0,
are used.
Example:
-1n+6 /* represents the first 6 elements of the list */
-4n+10 /* represents the 2nd, 6th, and 10th elements of the list */
If both
and
are 0,
the pseudo-class represents no element in the list.
6.1.
Informal Syntax Description
This section is non-normative.
When
is 0, the
An
part may be omitted
(unless the
part is already omitted).
When
An
is not included
and
is non-negative,
the
sign before
(when allowed)
may also be omitted.
In this case the syntax simplifies to just
Examples:
0n+5 /* represents the 5th element in the list */
5 /* same */
When
is 1 or -1,
the
may be omitted from the rule.
Examples:
The following notations are therefore equivalent:
1n+0 /* represents all elements in the list */
n+0 /* same */
n /* same */
If
is 0, then every
th element is picked.
In such a case,
the
+B
(or
-B
) part may be omitted
unless the
part is already omitted.
Examples:
2n+0 /* represents every even element in the list */
2n /* same */
When B is negative, its minus sign replaces the
sign.
Valid example:
3n-6
Invalid example:
3n + -6
Whitespace is permitted on either side of the
or
that separates the
An
and
parts when both are present.
Valid Examples with white space:
3n + 1
+3n - 2
-n+ 6
+6
Invalid Examples with white space:
3 n
+ 2n
+ 2
6.2.
The
type
The
An+B
notation was originally defined using a slightly different tokenizer than the rest of CSS,
resulting in a somewhat odd definition when expressed in terms of CSS tokens.
This section describes how to recognize the
An+B
notation in terms of CSS tokens
(thus defining the
type for CSS grammar purposes),
and how to interpret the CSS tokens to obtain values for
and
The
type is defined
(using the
Value Definition Syntax in the Values & Units spec
as:
odd
even
'+'
-n
'+'
'+'
-n
'+'
n-
-n-
['+'
'-']
'+'
n ['+'
'-']
-n ['+'
'-']
where:
is a
with its type flag set to "integer", and a unit that is an
ASCII case-insensitive
match for "n"
is a
with its type flag set to "integer", and a unit that is an
ASCII case-insensitive
match for "n-"
is a
with its type flag set to "integer", and a unit that is an
ASCII case-insensitive
match for "n-*", where "*" is a series of one or more
digits
is an
whose value is an
ASCII case-insensitive
match for "n-*", where "*" is a series of one or more
digits
is an
whose value is an
ASCII case-insensitive
match for "-n-*", where "*" is a series of one or more
digits
is a
with its type flag set to "integer"
is a
with its type flag set to "integer", and whose
representation
starts with "+" or "-"
is a
with its type flag set to "integer", and whose
representation
starts with a
digit
: When a plus sign (+) precedes an ident starting with "n", as in the cases marked above,
there must be no whitespace between the two tokens,
or else the tokens do not match the above grammar.
Whitespace is valid (and ignored) between any other two tokens.
The clauses of the production are interpreted as follows:
odd
is 2,
is 1.
even
is 2,
is 0.
is 0,
is the integer’s value.
'+'? n
-n
is the dimension’s value, 1, or -1, respectively.
is 0.
'+'?
is the dimension’s value or 1, respectively.
is the dimension’s unit or ident’s value, respectively,
with the first
code point
removed and the remainder interpreted as a base-10 number.
B is negative.
is -1.
is the ident’s value, with the first two
code points
removed and the remainder interpreted as a base-10 number.
B is negative.
'+'? n
-n
is the dimension’s value, 1, or -1, respectively.
is the integer’s value.
'+'? n-
-n-
is the dimension’s value, 1, or -1, respectively.
is the negation of the integer’s value.
['+' | '-']
'+'? n ['+' | '-']
-n ['+' | '-']
is the dimension’s value, 1, or -1, respectively.
is the integer’s value.
If a
'-'
was provided between the two,
is instead the negation of the integer’s value.
7.
The Unicode-Range microsyntax
Some constructs,
such as the
unicode-range
descriptor for the
@font-face
rule,
need a way to describe one or more unicode code points.
The
production represents a range of one or more unicode code points.
Informally, the
production has three forms:
U+0001
Defines a range consisting of a single code point,
in this case the code point "1".
U+0001-00ff
Defines a range of codepoints between the first and the second value inclusive,
in this case the range between "1" and "ff" (255 in decimal) inclusive.
U+00??
Defines a range of codepoints where the "?" characters range over all
hex digits
in this case defining the same as the value
U+0000-00ff
In each form, a maximum of 6 digits is allowed for each hexadecimal number
(if you treat "?" as a hexadecimal digit).
7.1.
The
type
The
notation was originally defined as a primitive token in CSS,
but it is used very rarely,
and collides with legitimate
s in confusing ways.
This section describes how to recognize the
notation
in terms of existing CSS tokens,
and how to interpret it as a range of unicode codepoints.
What are the confusing collisions?
For example, in the CSS
u + a { color: green; }
the intended meaning is that an
element
following a
element
should be colored green.
Whitespace is not normally required between combinators
and the surrounding selectors,
so it
should
be equivalent to minify it to
u+a{color:green;}
With any other combinator, the two pieces of CSS would be equivalent,
but due to the previous existence of a specialized unicode-range token,
the selector portion of the minified code now contains a unicode-range,
not two idents and a combinator.
It thus fails to match the Selectors grammar,
and the rule is thrown out as invalid.
(This example is taken from a real-world bug reported to Firefox.)
Note:
The syntax described here is intentionally very low-level,
and geared toward implementors.
Authors should instead read the informal syntax description in the previous section,
as it contains all information necessary to use
and is actually readable.
The
type is defined
(using the
Value Definition Syntax in the Values & Units spec
) as:
u '+'
'?'
'?'
'?'
u '+' '?'
In this production,
no whitespace can occur between any of the tokens.
The
production represents a range of one or more contiguous unicode code points
as a
start value
and an
end value
which are non-negative integers.
To interpret the production above into a range,
execute the following steps in order:
Skipping the first
token,
concatenate the
representations
of all the tokens in the production together.
Let this be
text
If the first character of
text
is U+002B PLUS SIGN,
consume it.
Otherwise,
this is an invalid
and this algorithm must exit.
Consume as many
hex digits
from
text
as possible.
then consume as many U+003F QUESTION MARK (?)
code points
as possible.
If zero
code points
were consumed,
or more than six
code points
were consumed,
this is an invalid
and this algorithm must exit.
If any U+003F QUESTION MARK (?)
code points
were consumed, then:
If there are any
code points
left in
text
this is an invalid
and this algorithm must exit.
Interpret the consumed
code points
as a hexadecimal number,
with the U+003F QUESTION MARK (?)
code points
replaced by U+0030 DIGIT ZERO (0)
code points
This is the
start value
Interpret the consumed
code points
as a hexadecimal number again,
with the U+003F QUESTION MARK (?)
code points
replaced by U+0046 LATIN CAPITAL LETTER F (F)
code points
This is the
end value
Exit this algorithm.
Otherwise, interpret the consumed
code points
as a hexadecimal number.
This is the
start value
If there are no
code points
left in
text
The
end value
is the same as the
start value
Exit this algorithm.
If the next
code point
in
text
is U+002D HYPHEN-MINUS (-),
consume it.
Otherwise,
this is an invalid
and this algorithm must exit.
Consume as many
hex digits
as possible from
text
If zero
hex digits
were consumed,
or more than 6
hex digits
were consumed,
this is an invalid
and this algorithm must exit.
If there are any
code points
left in
text
this is an invalid
and this algorithm must exit.
Interpret the consumed
code points
as a hexadecimal number.
This is the
end value
To determine what codepoints the
represents:
If
end value
is greater than the
maximum allowed code point
the
is invalid and a syntax error.
If
start value
is greater than
end value
the
is invalid and a syntax error.
Otherwise, the
represents a contiguous range of codepoints from
start value
to
end value
, inclusive.
Note:
The syntax of
is intentionally fairly wide;
its patterns capture every possible token sequence
that the informal syntax can generate.
However, it requires no whitespace between its constituent tokens,
which renders it fairly safe to use in practice.
Even grammars which have a
followed by a
or
(which might appear to be ambiguous
if an author specifies the
with the ''u
'' clause)
are actually quite safe,
as an author would have to intentionally separate the
and the
with a comment rather than whitespace
for it to be ambiguous.
Thus, while it’s
possible
for authors to write things that are parsed in confusing ways,
the actual code they’d have to write to cause the confusion is, itself, confusing and rare.
8.
Defining Grammars for Rules and Other Values
The
Values
spec defines how to specify a grammar for properties.
This section does the same, but for rules.
Just like in property grammars,
the notation
refers to the "foo" grammar term,
assumed to be defined elsewhere.
Substituting the
for its definition results in a semantically identical grammar.
Several types of tokens are written literally, without quotes:
s (such as
auto
disc
, etc), which are simply written as their value.
s, which are written as an @ character followed by the token’s value, like
@media
s, which are written as the function name followed by a ( character, like
translate(
The
(written as
),
(written as
),
(written as
),
<(-token>
<)-token>
<{-token>
, and
<}-token>
s.
Tokens match if their value is a match for the value defined in the grammar.
Unless otherwise specified, all matches are
ASCII case-insensitive
Note:
Although it is possible, with
escaping
to construct an
whose value ends with
or starts with
such a tokens is not a
or an
and does not match corresponding grammar definitions.
s are written with their value enclosed in single quotes.
For example, a
containing the "+"
code point
is written as
'+'
Similarly, the
<[-token>
and
<]-token>
s must be written in single quotes,
as they’re used by the syntax of the grammar itself to group clauses.
is never indicated in the grammar;
s are allowed before, after, and between any two tokens,
unless explicitly specified otherwise in prose definitions.
(For example, if the prelude of a rule is a selector,
whitespace is significant.)
When defining a function or a block,
the ending token must be specified in the grammar,
but if it’s not present in the eventual token stream,
it still matches.
For example, the syntax of the
translateX()
function is:
translateX(
However, the stylesheet may end with the function unclosed, like:
.foo { transform: translate(50px
The CSS parser parses this as a style rule containing one declaration,
whose value is a function named "translate".
This matches the above grammar,
even though the ending token didn’t appear in the token stream,
because by the time the parser is finished,
the presence of the ending token is no longer possible to determine;
all you have is the fact that there’s a block and a function.
8.1.
Defining Block Contents: the
, and
productions
The CSS parser is agnostic as to the contents of blocks,
such as those that come at the end of some at-rules.
Defining the generic grammar of the blocks in terms of tokens is non-trivial,
but there are dedicated and unambiguous algorithms defined for parsing this.
The
production represents the contents of a
style rule’s
block.
It may only be used in grammars as the sole value in a block,
and represents that the contents of the block must be parsed using the
consume a style block’s contents
algorithm.
The
production represents a list of declarations.
It may only be used in grammars as the sole value in a block,
and represents that the contents of the block must be parsed using the
consume a list of declarations
algorithm.
Similarly, the
production represents a list of rules,
and may only be used in grammars as the sole value in a block.
It represents that the contents of the block must be parsed using the
consume a list of rules
algorithm.
Finally, the
production represents a list of rules.
It is identical to
except that blocks using it default to accepting all rules
that aren’t otherwise limited to a particular context.
All four of these productions are pretty similar to each other,
so this table summarizes what they accept
and lists some example instances of each:
Allows
declarations
Allows
nested style rules
Allow arbitrary
qualified rules
Allows
at-rules
Examples
style rules
@nest
nested conditional group rules
@font
@counter-style
@page
@keyframes
child rules
@keyframes
@font-feature-values
stylesheets, non-nested
conditional group rules
If a given context is
only
intended to accept
at-rules
such as in
@font-features-values
it doesn’t actually matter which production is used,
but
is preferred for its more straightforward name.
For example, the
@font-face
rule is defined to have an empty prelude,
and to contain a list of declarations.
This is expressed with the following grammar:
@font-face {
This is a complete and sufficient definition of the rule’s grammar.
For another example,
@keyframes
rules are more complex,
interpreting their prelude as a name and containing keyframes rules in their block
Their grammar is:
@keyframes
For rules that use
or
the spec for the rule must define which properties, descriptors, and/or at-rules are valid inside the rule;
this may be as simple as saying "The @foo rule accepts the properties/descriptors defined in this specification/section.",
and extension specs may simply say "The @foo rule additionally accepts the following properties/descriptors.".
Any declarations or at-rules found inside the block that are not defined as valid
must be removed from the rule’s value.
Within a
or
!important
is automatically invalid on any descriptors.
If the rule accepts properties,
the spec for the rule must define whether the properties interact with the cascade,
and with what specificity.
If they don’t interact with the cascade,
properties containing
!important
are automatically invalid;
otherwise using
!important
is valid
and causes the declaration to be
important
for the purposes of the
cascade
See
[CSS-CASCADE-3]
For example, the grammar for
@font-face
in the previous example must,
in addition to what is written there,
define that the allowed declarations are the descriptors defined in the Fonts spec.
For rules that use
the spec for the rule must define what types of rules are valid inside the rule,
same as
and unrecognized rules must similarly be removed from the rule’s value.
For example, the grammar for
@keyframes
in the previous example must,
in addition to what is written there,
define that the only allowed rules are
s,
which are defined as:
Keyframe rules, then,
must further define that they accept as declarations all animatable CSS properties,
plus the
animation-timing-function
property,
but that they do not interact with the cascade.
For rules that use
all rules are allowed by default,
but the spec for the rule may define what types of rules are
invalid
inside the rule.
For example, the
@media
rule accepts anything that can be placed in a stylesheet,
except more
@media
rules.
As such, its grammar is:
@media
It additionally defines a restriction that the
can not contain
@media
rules,
which causes them to be dropped from the outer rule’s value if they appear.
8.2.
Defining Arbitrary Contents: the
and
productions
In some grammars,
it is useful to accept any reasonable input in the grammar,
and do more specific error-handling on the contents manually
(rather than simply invalidating the construct,
as grammar mismatches tend to do).
For example,
custom properties
allow any reasonable value,
as they can contain arbitrary pieces of other CSS properties,
or be used for things that aren’t part of existing CSS at all.
For another example, the
production in Media Queries
defines the bounds of what future syntax MQs will allow,
and uses special logic to deal with "unknown" values.
To aid in this, two additional productions are defined:
The
production matches
any
sequence of one or more tokens,
so long as the sequence does not contain
unmatched
<)-token>
<]-token>
, or
<}-token>
or top-level
tokens or
tokens with a value of "!".
It represents the entirety of what a valid declaration can have as its value.
The
production is identical to
but also allows top-level
tokens
and
tokens with a value of "!".
It represents the entirety of what valid CSS can be in any context.
9.
CSS stylesheets
To
parse a CSS stylesheet
first
parse a stylesheet
Interpret all of the resulting top-level
qualified rules
as
style rules
, defined below.
If any style rule is
invalid
or any at-rule is not recognized or is invalid according to its grammar or context,
it’s a
parse error
Discard that rule.
9.1.
Style rules
style rule
is a
qualified rule
that associates a
selector list
with a list of property declarations
and possibly a list of nested rules.
They are also called
rule sets
in
[CSS2]
CSS Cascading and Inheritance
[CSS-CASCADE-3]
defines how the declarations inside of style rules participate in the cascade.
The prelude of the qualified rule is
parsed
as a
If this returns failure,
the entire style rule is
invalid
The content of the qualified rule’s block is parsed as a
style block’s contents
Unless defined otherwise by another specification or a future level of this specification,
at-rules in that list are
invalid
and must be ignored.
Note:
[CSS-NESTING-1]
defines that
@nest
and
conditional group rules
are allowed inside of
style rules
Declarations for an unknown CSS property
or whose value does not match the syntax defined by the property are
invalid
and must be ignored.
The validity of the style rule’s contents have no effect on the validity of the style rule itself.
Unless otherwise specified, property names are
ASCII case-insensitive
Note:
The names of Custom Properties
[CSS-VARIABLES]
are case-sensitive.
Qualified rules
at the top-level of a CSS stylesheet are
style rules
Qualified rules in other contexts may or may not be style rules,
as defined by the context.
For example, qualified rules inside
@media
rules
[CSS3-CONDITIONAL]
are style rules,
but qualified rules inside
@keyframes
rules
[CSS3-ANIMATIONS]
are not.
9.2.
At-rules
An
at-rule
is a rule that begins with an at-keyword,
and can thus be distinguished from
style rules
in the same context.
At-rules
are used to:
group and structure style rules and other at-rules
such as in
conditional group rules
declare style information that is not associated with a particular element,
such as defining
counter styles
manage syntactic constructs
such as
imports
and
namespaces
keyword mappings
and serve other miscellaneous purposes not served by a
style rule
At-rules take many forms, depending on the specific rule and its purpose,
but broadly speaking there are two kinds:
statement at-rules
which are simpler constructs that end in a semicolon,
and
block at-rules
which end in a
{}-block
that can contain nested
qualified rules
at-rules
, or
declarations
Block at-rules
will typically contain a collection of
(generic or
at-rule
–specific)
at-rules
qualified rules
, and/or
descriptor declarations
subject to limitations defined by the
at-rule
Descriptors
are similar to
properties
(and are declared with the same syntax)
but are associated with a particular type of
at-rule
rather than with elements and boxes in the tree.
9.3.
The
@charset
Rule
The algorithm used to
determine the fallback encoding
for a stylesheet
looks for a specific byte sequence as the very first few bytes in the file,
which has the syntactic form of an
at-rule
named "@charset".
However, there is no actual
at-rule
named
@charset
When a stylesheet is actually parsed,
any occurrences of an
@charset
rule must be treated as an unrecognized rule,
and thus dropped as invalid when the stylesheet is grammar-checked.
Note:
In CSS 2.1,
@charset
was a valid rule.
Some legacy specs may still refer to a
@charset
rule,
and explicitly talk about its presence in the stylesheet.
10.
Serialization
The tokenizer described in this specification does not produce tokens for comments,
or otherwise preserve them in any way.
Implementations may preserve the contents of comments and their location in the token stream.
If they do, this preserved information must have no effect on the parsing step.
This specification does not define how to serialize CSS in general,
leaving that task to the
[CSSOM]
and individual feature specifications.
In particular, the serialization of comments and whitespace is not defined.
The only requirement for serialization is that it must "round-trip" with parsing,
that is, parsing the stylesheet must produce the same data structures as
parsing, serializing, and parsing again,
except for consecutive
s,
which may be collapsed into a single token.
Note:
This exception can exist because
CSS grammars always interpret any amount of whitespace as identical to a single space.
To satisfy this requirement:
containing U+005C REVERSE SOLIDUS (\)
must be serialized as U+005C REVERSE SOLIDUS
followed by a
newline
(The tokenizer only ever emits such a token followed by a
that starts with a newline.)
with the "unrestricted" type flag may not need
as much escaping as the same token with the "id" type flag.
The unit of a
may need escaping
to disambiguate with scientific notation.
For any consecutive pair of tokens,
if the first token shows up in the row headings of the following table,
and the second token shows up in the column headings,
and there’s a ✗ in the cell denoted by the intersection of the chosen row and column,
the pair of tokens must be serialized with a comment between them.
If the tokenizer preserves comments,
the preserved comment should be used;
otherwise, an empty comment (
/**/
) must be inserted.
(Preserved comments may be reinserted even if the following tables don’t require a comment between two tokens.)
Single characters in the row and column headings represent a
with that value,
except for "
",
which represents a
(-token
ident
function
url
bad url
number
percentage
dimension
CDC
ident
at-keyword
hash
dimension
number
10.1.
Serializing
To
serialize an
value
with integer values
and
If
is zero,
return the serialization of
Otherwise, let
result
initially be an empty
string
is
Append "n" to
result
is
-1
Append "-n" to
result
is non-zero
Serialize
and append it to
result
then append "n" to
result
is greater than zero
Append "+" to
result
then append the serialization of
to
result
is less than zero
Append the serialization of
to
result
Return
result
11.
Privacy and Security Considerations
This specification introduces no new privacy concerns.
This specification improves security, in that CSS parsing is now unambiguously defined for all inputs.
Insofar as old parsers, such as whitelists/filters, parse differently from this specification,
they are somewhat insecure,
but the previous parsing specification left a lot of ambiguous corner cases which browsers interpreted differently,
so those filters were potentially insecure already,
and this specification does not worsen the situation.
12.
Changes
This section is non-normative.
12.1.
Changes from the 16 August 2019 Candidate Recommendation
The following substantive changes were made:
Added a new
§ 5.3.2 Parse A Comma-Separated List According To A CSS Grammar
algorithm.
Added a new
§ 5.3.7 Parse a style block’s contents
algorithm
and a corresponding
production,
and defined that
style rules
use it.
Aligned
parse a stylesheet
with the Fetch-related shenanigans.
(See
commit
.)
To
parse a stylesheet
from an
input
given an optional
url
location
...
Create a new stylesheet
, with its
location
set to
location
(or null, if
location
was not passed).
...
The following editorial changes were made:
Added
§ 9.2 At-rules
to provide definitions for
at-rules
statement at-rules
block at-rules
, and
descriptors
5633
Improved the definition text for
declaration
and added definitions for
property declarations
and
descriptor declarations
Switched to consistently refer to
ident sequence
rather than sometimes using the term “name”.
Explicitly named several of the pre-tokenizing processes,
and explicitly referred to them in the parsing entry points
(rather than relying on a blanket "do X at the start of these algorithms" statement).
Added more entries to the "put a comment between them" table,
to properly handle the fact that idents can now start with
--
6874
12.2.
Changes from the 20 February 2014 Candidate Recommendation
The following substantive changes were made:
Removed
, in favor of creating a
production.
url() functions that contain a string are now parsed as normal
s.
url() functions that contain "raw" URLs are still specially parsed as
s.
Fixed a bug in the "Consume a URL token" algorithm,
where it didn’t consume the quote character starting a string before attempting to consume the string.
Fixed a bug in several of the parser algorithms
related to the current/next input token and things getting consumed early/late.
Fix several bugs in the tokenization and parsing algorithms.
Change the definition of ident-like tokens to allow "--" to start an ident.
As part of this, rearrange the ordering of the clauses in the "-" step of
consume a token
so that
s are recognized as such instead of becoming a
--
Don’t serialize the digit in an
when A is 1 or -1.
Define all tokens to have a
representation
Fixed minor bug in
check if two code points are a valid escape
followed by an EOF is now correctly reported as
not
a valid escape.
A final
in a stylesheet now just emits itself as a
@charset is no longer a valid CSS rule (there’s just an encoding declaration that
looks
like a rule named @charset)
Trimmed whitespace from the beginning/ending of a declaration’s value during parsing.
Removed the Selectors-specific tokens, per WG resolution.
Filtered
surrogates
from the input stream, per WG resolution.
Now the entire specification operates only on
scalar values
The following editorial changes were made:
The "Consume a string token" algorithm was changed to allow calling it without specifying an explicit ending token,
so that it uses the current input token instead.
The three call-sites of the algorithm were changed to use that form.
Minor editorial restructuring of algorithms.
Added the
parse
and
parse a comma-separated list of component values
API entry points.
Added the
and
productions.
Removed "code point" and "surrogate code point" in favor of the identical definitions in the Infra Standard.
Clarified on every range that they are inclusive.
Added a column to the comment-insertion table to handle a number token appearing next to a "%" delim token.
A Disposition of Comments is available.
12.3.
Changes from the 5 November 2013 Last Call Working Draft
The
Serialization
section has been rewritten
to make only the "round-trip" requirement normative,
and move the details of how to achieve it into a note.
Some corner cases in these details have been fixed.
[ENCODING]
has been added to the list of normative references.
It was already referenced in normative text before,
just not listed as such.
In the algorithm to
determine the fallback encoding
of a stylesheet,
limit the
@charset
byte sequence to 1024 bytes.
This aligns with what HTML does for
and makes sure the size of the sequence is bounded.
This only makes a difference with leading or trailing whitespace
in the encoding label:
@charset "
(lots of whitespace)
utf-8";
12.4.
Changes from the 19 September 2013 Working Draft
The concept of
environment encoding
was added.
The behavior does not change,
but some of the definitions should be moved to the relevant specs.
12.5.
Changes from CSS 2.1 and Selectors Level 3
Note:
The point of this spec is to match reality;
changes from CSS2.1 are nearly always because CSS 2.1 specified something that doesn’t match actual browser behavior,
or left something unspecified.
If some detail doesn’t match browsers,
please let me know
as it’s almost certainly unintentional.
Changes in decoding from a byte stream:
Only detect
@charset
rules in ASCII-compatible byte patterns.
Ignore
@charset
rules that specify an ASCII-incompatible encoding,
as that would cause the rule itself to not decode properly.
Refer to
[ENCODING]
rather than the IANA registry for character encodings.
Tokenization changes:
Any U+0000 NULL
code point
in the CSS source is replaced with U+FFFD REPLACEMENT CHARACTER.
Any hexadecimal escape sequence such as
\0
that evaluates to zero
produce U+FFFD REPLACEMENT CHARACTER rather than U+0000 NULL.
The definition of
non-ASCII code point
was changed
to be consistent with every definition of ASCII.
This affects
code points
U+0080 to U+009F,
which are now
ident code points
rather than
s,
like the rest of
non-ASCII code points
Tokenization does not emit COMMENT or BAD_COMMENT tokens anymore.
BAD_COMMENT is now considered the same as a normal token (not an error).
Serialization
is responsible
for inserting comments as necessary between tokens that need to be separated,
e.g. two consecutive
s.
The
was removed,
as it was low value and occasionally actively harmful.
u+a { font-weight: bold; }
was an invalid selector, for example...)
Instead, a
production was added,
based on token patterns.
It is technically looser than what 2.1 allowed
(any number of digits and ? characters),
but not in any way that should impact its use in practice.
Apply the
EOF error handling rule
in the tokenizer
and emit normal
and
rather than BAD_STRING or BAD_URI
on EOF.
The BAD_URI token (now
) is "self-contained".
In other words, once the tokenizer realizes it’s in a
rather than a
it just seeks forward to look for the closing ),
ignoring everything else.
This behavior is simpler than treating it like a
and paying attention to opened blocks and such.
Only WebKit exhibits this behavior,
but it doesn’t appear that we’ve gotten any compat bugs from it.
The
has been added.
, and
have been changed
to include the preceding +/- sign as part of their value
(rather than as a separate
that needs to be manually handled every time the token is mentioned in other specs).
The only consequence of this is that comments can no longer be inserted between the sign and the number.
Scientific notation is supported for numbers/percentages/dimensions to match SVG,
per WG resolution.
Hexadecimal escape for
surrogate
now emit a replacement character rather than the surrogate.
This allows implementations to safely use UTF-16 internally.
Parsing changes:
Any list of declarations now also accepts at-rules, like
@page
per WG resolution.
This makes a difference in error handling
even if no such at-rules are defined yet:
an at-rule, valid or not, ends at a {} block without a
and lets the next declaration begin.
The handling of some miscellaneous "special" tokens
(like an unmatched
<}-token>
showing up in various places in the grammar
has been specified with some reasonable behavior shown by at least one browser.
Previously, stylesheets with those tokens in those places just didn’t match the stylesheet grammar at all,
so their handling was totally undefined.
Specifically:
[] blocks, () blocks and functions can now contain {} blocks,
s or
Qualified rule preludes can now contain semicolons
Qualified rule and at-rule preludes can now contain
An+B
changes from Selectors Level 3
[SELECT]
The
An+B
microsyntax has now been formally defined in terms of CSS tokens,
rather than with a separate tokenizer.
This has resulted in minor differences:
In some cases, minus signs or digits can be escaped
(when they appear as part of the unit of a
or
).
Acknowledgments
Thanks for feedback and contributions from
Anne van Kesteren,
David Baron,
Elika J. Etemad (fantasai),
Henri Sivonen,
Johannes Koch,
呂康豪 (Kang-Hao Lu),
Marc O’Morain,
Raffaello Giulietti,
Simon Pieter,
Tyler Karaszewski,
and Zack Weinberg.
Conformance
Document conventions
Conformance requirements are expressed with a combination of
descriptive assertions and RFC 2119 terminology. The key words “MUST”,
“MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
“RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
document are to be interpreted as described in RFC 2119.
However, for readability, these words do not appear in all uppercase
letters in this specification.
All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes.
[RFC2119]
Examples in this specification are introduced with the words “for example”
or are set apart from the normative text with
class="example"
like this:
This is an example of an informative example.
Informative notes begin with the word “Note” and are set apart from the
normative text with
class="note"
, like this:
Note, this is an informative note.
Advisements are normative sections styled to evoke special attention and are
set apart from other normative text with
, like
this:
UAs MUST provide an accessible alternative.
Conformance classes
Conformance to this specification
is defined for three conformance classes:
style sheet
CSS
style sheet
renderer
UA
that interprets the semantics of a style sheet and renders
documents that use them.
authoring tool
UA
that writes a style sheet.
A style sheet is conformant to this specification
if all of its statements that use syntax defined in this module are valid
according to the generic CSS grammar and the individual grammars of each
feature defined in this module.
A renderer is conformant to this specification
if, in addition to interpreting the style sheet as defined by the
appropriate specifications, it supports all the features defined
by this specification by parsing them correctly
and rendering the document accordingly. However, the inability of a
UA to correctly render a document due to limitations of the device
does not make the UA non-conformant. (For example, a UA is not
required to render color on a monochrome monitor.)
An authoring tool is conformant to this specification
if it writes style sheets that are syntactically correct according to the
generic CSS grammar and the individual grammars of each feature in
this module, and meet all other conformance requirements of style sheets
as described in this module.
Partial implementations
So that authors can exploit the forward-compatible parsing rules to
assign fallback values, CSS renderers
must
treat as invalid (and
ignore
as appropriate
) any at-rules, properties, property values, keywords,
and other syntactic constructs for which they have no usable level of
support. In particular, user agents
must not
selectively
ignore unsupported component values and honor supported values in a single
multi-value property declaration: if any value is considered invalid
(as unsupported values must be), CSS requires that the entire declaration
be ignored.
Implementations of Unstable and Proprietary Features
To avoid clashes with future stable CSS features,
the CSSWG recommends
following best practices
for the implementation of
unstable
features and
proprietary extensions
to CSS.
Non-experimental implementations
Once a specification reaches the Candidate Recommendation stage,
non-experimental implementations are possible, and implementors should
release an unprefixed implementation of any CR-level feature they
can demonstrate to be correctly implemented according to spec.
To establish and maintain the interoperability of CSS across
implementations, the CSS Working Group requests that non-experimental
CSS renderers submit an implementation report (and, if necessary, the
testcases used for that implementation report) to the W3C before
releasing an unprefixed implementation of any CSS features. Testcases
submitted to W3C are subject to review and correction by the CSS
Working Group.
Further information on submitting testcases and implementation reports
can be found from on the CSS Working Group’s website at
Questions should be directed to the
public-css-testsuite@w3.org
mailing list.
CR exit criteria
For this specification to be advanced to Proposed Recommendation,
there must be at least two independent, interoperable implementations
of each feature. Each feature may be implemented by a different set of
products, there is no requirement that all features be implemented by
a single product. For the purposes of this criterion, we define the
following terms:
independent
each implementation must be developed by a
different party and cannot share, reuse, or derive from code
used by another qualifying implementation. Sections of code that
have no bearing on the implementation of this specification are
exempt from this requirement.
interoperable
passing the respective test case(s) in the
official CSS test suite, or, if the implementation is not a Web
browser, an equivalent test. Every relevant test in the test
suite should have an equivalent test created if such a user
agent (UA) is to be used to claim interoperability. In addition
if such a UA is to be used to claim interoperability, then there
must one or more additional UAs which can also pass those
equivalent tests in the same way for the purpose of
interoperability. The equivalent tests must be made publicly
available for the purposes of peer review.
implementation
a user agent which:
implements the specification.
is available to the general public. The implementation may
be a shipping product or other publicly available version
(i.e., beta version, preview release, or "nightly build").
Non-shipping product releases must have implemented the
feature(s) for a period of at least one month in order to
demonstrate stability.
is not experimental (i.e., a version specifically designed
to pass the test suite and is not intended for normal usage
going forward).
The specification will remain Candidate Recommendation for at least
six months.
Index
Terms defined by this specification
, in § 6.2
An+B
, in § 6
, in § 8.2
are a valid escape
, in § 4.3.8
, in § 4
at-rule
, in § 9.2
, in § 4
, in § 4
()-block
, in § 5
[]-block
, in § 5
{}-block
, in § 5
block at-rule
, in § 9.2
, in § 4
, in § 4
@charset
, in § 9.3
check if three code points would start an ident sequence
, in § 4.3.9
check if three code points would start a number
, in § 4.3.10
check if two code points are a valid escape
, in § 4.3.8
, in § 4
, in § 4
component value
, in § 5
consume a component value
, in § 5.4.7
consume a declaration
, in § 5.4.6
consume a function
, in § 5.4.9
consume a list of declarations
, in § 5.4.5
consume a list of rules
, in § 5.4.1
consume an at-rule
, in § 5.4.2
consume an escaped code point
, in § 4.3.7
consume an ident-like token
, in § 4.3.4
consume an ident sequence
, in § 4.3.11
consume a number
, in § 4.3.12
consume a numeric token
, in § 4.3.3
consume a qualified rule
, in § 5.4.3
consume a simple block
, in § 5.4.8
consume a string token
, in § 4.3.5
consume a style block’s contents
, in § 5.4.4
consume a token
, in § 4.3.1
consume a url token
, in § 4.3.6
consume comments
, in § 4.3.2
consume the next input token
, in § 5.2
consume the remnants of a bad url
, in § 4.3.14
convert a string to a number
, in § 4.3.13
CSS ident sequence
, in § 4.2
current input code point
, in § 4.2
current input token
, in § 5.2
, in § 6.2
declaration
, in § 5
, in § 8.1
, in § 8.2
decode bytes
, in § 3.2
, in § 4
descriptor
, in § 9.2
descriptor declarations
, in § 5
determine the fallback encoding
, in § 3.2
digit
, in § 4.2
, in § 4
ending token
, in § 5.4.8
environment encoding
, in § 3.2
EOF code point
, in § 4.2
, in § 5.2
escaping
, in § 2.1
filter code points
, in § 3.3
filtered code points
, in § 3.3
function
, in § 5
, in § 4
, in § 4
hex digit
, in § 4.2
ident code point
, in § 4.2
ident sequence
, in § 4.2
ident-start code point
, in § 4.2
, in § 4
ignored
, in § 2.2
input stream
, in § 3.3
, in § 6.2
invalid
, in § 2.2
letter
, in § 4.2
lowercase letter
, in § 4.2
maximum allowed code point
, in § 4.2
name-start code point
, in § 4.2
, in § 6.2
, in § 6.2
, in § 6.2
, in § 6.2
newline
, in § 4.2
next input code point
, in § 4.2
next input token
, in § 5.2
non-ASCII code point
, in § 4.2
non-printable code point
, in § 4.2
normalize
, in § 5.3
normalize into a token stream
, in § 5.3
, in § 4
parse
, in § 5.3.1
parse a comma-separated list according to a CSS grammar
, in § 5.3.2
parse a comma-separated list of component values
, in § 5.3.11
parse a component value
, in § 5.3.9
parse a CSS stylesheet
, in § 9
parse a declaration
, in § 5.3.6
parse a list
, in § 5.3.2
parse a list of component values
, in § 5.3.10
parse a list of declarations
, in § 5.3.8
parse a list of rules
, in § 5.3.4
parse a rule
, in § 5.3.5
parse a style block’s contents
, in § 5.3.7
parse a stylesheet
, in § 5.3.3
parse error
, in § 3
parse something according to a CSS grammar
, in § 5.3.1
parsing a list
, in § 5.3.2
, in § 4
preserved tokens
, in § 5
property declarations
, in § 5
qualified rule
, in § 5
reconsume the current input code point
, in § 4.2
reconsume the current input token
, in § 5.2
representation
, in § 4.2
, in § 8.1
, in § 4
serialize an
, in § 10.1
, in § 6.2
, in § 6.2
simple block
, in § 5
starts with an ident sequence
, in § 4.3.9
starts with a number
, in § 4.3.10
starts with a valid escape
, in § 4.3.8
start with an ident sequence
, in § 4.3.9
start with a number
, in § 4.3.10
statement at-rule
, in § 9.2
, in § 4
, in § 8.1
style rule
, in § 9.1
, in § 8.1
<(-token>
, in § 4
<)-token>
, in § 4
<[-token>
, in § 4
<]-token>
, in § 4
<{-token>
, in § 4
<}-token>
, in § 4
tokenization
, in § 4
tokenize
, in § 4
uppercase letter
, in § 4.2
, in § 7
, in § 4
whitespace
, in § 4.2
, in § 4
would start an ident sequence
, in § 4.3.9
would start a number
, in § 4.3.10
Referenced in:
2.
Description of CSS’s Syntax
3.2.
The input byte stream
Referenced in:
5.
Parsing
9.2.
At-rules
Referenced in:
8.1.
Defining Block Contents: the
Referenced in:
8.1.
Defining Block Contents: the
Referenced in:
5.3.1.
Parse something according to a CSS grammar
Referenced in:
2.
Description of CSS’s Syntax
Referenced in:
2.
Description of CSS’s Syntax
Referenced in:
8.1.
Defining Block Contents: the
Referenced in:
9.2.
At-rules
Referenced in:
8.1.
Defining Block Contents: the
Referenced in:
7.
The Unicode-Range microsyntax
Referenced in:
7.
The Unicode-Range microsyntax
8.1.
Defining Block Contents: the
(2)
Referenced in:
8.1.
Defining Block Contents: the
9.1.
Style rules
Referenced in:
8.1.
Defining Block Contents: the
Referenced in:
8.1.
Defining Block Contents: the