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, in speech, etc.
Status of this document
This section describes the status of this document at the time of
its publication. Other documents may supersede this document. A list of
current W3C publications and the latest revision of this technical report
can be found in the W3C technical reports
index at http://www.w3.org/TR/.
Publication as a Working Draft does not imply endorsement by the W3C
Membership. 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.
The (archived) public
mailing list www-style@w3.org (see
instructions) is preferred
for discussion of this specification. When sending e-mail, please put the
text “css-syntax” in the subject, preferably like this:
“[css-syntax] …summary of comment…”
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 qualified rules,
which are usually style rules that apply CSS properties to elements,
and at-rules,
which define special processing rules or values for the CSS document.
A 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 <a> elements that are children of a <p> element.
"color: blue;" is a declaration specifying that,
for the elements that match the selector,
their color property should have the value blue.
Similiarly, 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.
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 @importat-rule is a simple statement.
After its name, it takes a single string or url() function to indicate the stylesheet that it should import.
The @pageat-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 @mediaat-rule begins with a media type
and a list of optional media queries.
Its block contains entire rules,
which are only applied when the @medias conditions are fulfilled.
Property names and at-rule names are always identifiers,
which have to start with a letter or a hyphen followed by a letter,
and then can contain letters, numbers, hyphens, or underscores.
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 identifier or quoted string
by escaping it.
CSS escape sequences start with a backslash (\), and continue with:
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 identifier 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 <at-keyword-token> 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 <semicolon-token> 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.
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 a 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
may be initially seen by the user agent as a stream of bytes
(typically coming over the network or from the local file system).
The bytes encode the code points according to a particular character encoding,
which the user agent must use to decode the bytes into code points.
To decode the stream of bytes into a stream of code points,
UAs must follow these steps.
If HTTP or equivalent protocol defines an encoding (e.g. via the charset parameter of the Content-Type header),
get an encoding for the specified value.
If that does not return failure,
use the return value as the fallback encoding.
Otherwise, check the byte stream. If the first several bytes match the hex sequence
40 63 68 61 72 73 65 74 20 22 (not 22)* 22 3B
then get an encoding for the sequence of (not 22)* bytes,
decoded per windows-1252.
Note: Anything ASCII-compatible will do, so using windows-1252 is fine.
Note: The byte sequence above,
when decoded as ASCII,
is the string "@charset "…";",
where the "…" is the sequence of bytes corresponding to the encoding’s name.
If the return value was utf-16 or utf-16be,
use utf-8 as the fallback encoding;
if it was anything else except failure,
use the return value as the fallback encoding.
Note: This mimics HTML <meta> behavior.
Otherwise, get an encoding for the value of the charset attribute on the <link> element or <?xml-stylesheet?> processing instruction that caused the style sheet to be included, if any.
If that does not return failure,
use the return value as the fallback encoding.
Otherwise, if the referring style sheet or document has an encoding,
use that as the fallback encoding.
Otherwise, use utf-8 as the fallback encoding.
Then, decode the byte stream using the fallback encoding.
Note: the decode algorithm lets the byte order mark (BOM) take precedence,
hence the usage of the term "fallback" above.
Anne says that steps 3/4 should be an input to this algorithm from the specs that define importing stylesheet,
to make the algorithm as a whole cleaner.
Perhaps abstract it into the concept of an "environment charset" or something?
Should we only take the charset from the referring document if it’s same-origin?
3.3
Preprocessing the input stream
The input stream consists of the code points
pushed into it as the input byte stream is decoded.
Before sending the input stream to the tokenizer,
implementations must make the following code point substitutions:
Replace any U+000D CARRIAGE RETURN (CR) code point,
U+000C FORM FEED (FF) code point,
or pairs of U+000D CARRIAGE RETURN (CR) followed by U+000A LINE FEED (LF)
by a single U+000A LINE FEED (LF) code point.
Replace any U+0000 NULL code point with U+FFFD REPLACEMENT CHARACTER (�).
4
Tokenization
Implementations must act as if they used the following algorithms to tokenize CSS.
To transform a stream of code points into a stream of tokens,
repeatedly consume a token
until an <EOF-token> is reached,
collecting the returned tokens into a stream.
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 pointsduring parsing,
if so desired.
The output of the tokenization step is a stream of zero or more of the following tokens:
<ident-token>,
<function-token>,
<at-keyword-token>,
<hash-token>,
<string-token>,
<bad-string-token>,
<url-token>,
<bad-url-token>,
<delim-token>,
<number-token>,
<percentage-token>,
<dimension-token>,
<unicode-range-token>,
<include-match-token>,
<dash-match-token>,
<prefix-match-token>,
<suffix-match-token>,
<substring-match-token>,
<column-token>,
<whitespace-token>,
<CDO-token>,
<CDC-token>,
<colon-token>,
<semicolon-token>,
<comma-token>,
<[-token>,
<]-token>,
<(-token>,
<)-token>,
<{-token>,
and <}-token>.
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.
Note: As a technical note,
the tokenizer defined here requires only three code points of look-ahead.
The tokens it produces are designed to allow Selectors to be parsed with one token of look-ahead,
and additional tokens may be added in the future to maintain this invariant.
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 <foo-token> represent tokens.
The rest are productions referred to by other diagrams.
comment
newline
whitespace
hex digit
escape
<whitespace-token>
ws*
<ident-token>
<function-token>
<at-keyword-token>
<hash-token>
<string-token>
<url-token>
url-unquoted
<number-token>
<dimension-token>
<percentage-token>
<unicode-range-token>
<include-match-token>
<dash-match-token>
<prefix-match-token>
<suffix-match-token>
<substring-match-token>
<column-token>
<CDO-token>
<CDC-token>
4.2
Definitions
This section defines several terms used during the tokenization phase.
code point
A Unicode code point.
Any value in the Unicode codespace; that is, the range of integers from 0 to (hexadecimal) 10FFFF.
next input code point
The first code point in the input stream that has not yet been consumed.
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
A code point between U+0030 DIGIT ZERO (0) and U+0039 DIGIT NINE (9).
hex digit
A digit,
or a code point between U+0041 LATIN CAPITAL LETTER A (A) and U+0046 LATIN CAPITAL LETTER F (F),
or a code point between U+0061 LATIN SMALL LETTER A (a) and U+0066 LATIN SMALL LETTER F (f).
uppercase letter
A code point between U+0041 LATIN CAPITAL LETTER A (A) and U+005A LATIN CAPITAL LETTER Z (Z).
lowercase letter
A code point between U+0061 LATIN SMALL LETTER A (a) and U+007A LATIN SMALL LETTER Z (z).
A code point between U+0000 NULL and U+0008 BACKSPACE,
or U+000B LINE TABULATION,
or a code point between U+000E SHIFT OUT and U+001F INFORMATION SEPARATOR ONE,
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
A newline, U+0009 CHARACTER TABULATION, or U+0020 SPACE.
If the next input code point is U+002A ASTERISK (*),
consume it
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.
Then consume a token
and return it.
Otherwise,
if the next input code point is U+0025 PERCENTAGE SIGN (%),
consume it.
Create a <percentage-token> with the same representation and value as the returned number,
and return it.
Otherwise,
create a <number-token> with the same representation, value, and type flag as the returned number,
and return it.
Otherwise,
if the next input code point is U+0028 LEFT PARENTHESIS ((),
consume it.
Create a <function-token>
with its value set to the returned string
and return it.
Otherwise,
create an <ident-token>
with its value set to the returned string
and return it.
This section describes how to consume a unicode-range token.
It returns a <unicode-range-token>.
Note: This algorithm assumes that the initial "u+" has been consumed,
and the next code point verified to be a hex digit or a "?".
Execute the following steps in order:
Consume as many hex digits as possible, but no more than 6.
If less than 6 hex digits were consumed,
consume as many U+003F QUESTION MARK (?) code points as possible,
but no more than enough to make the total of hex digits and U+003F QUESTION MARK (?) code points equal to 6.
If any U+003F QUESTION MARK (?) code points were consumed,
then:
Interpret the consumed code points as a hexadecimal number again,
with the U+003F QUESTION MARK (?) code point replaced by U+0046 LATIN CAPITAL LETTER F (F) code points.
This is the end of the range.
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 not be a newline or EOF.
It will return a code point.
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 code point,
or is greater than the maximum allowed code point,
return U+FFFD REPLACEMENT CHARACTER (�).
Otherwise, return the code point with that value.
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+005D REVERSE SOLIDUS (\),
return false.
4.3.9
Check if three code points would start an identifier
This section describes how to check if three code points would start an identifier.
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 point.
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.
This section describes how to consume a name 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 <ident-token>.
If that is the intended use,
ensure that the stream starts with an identifier
before calling this algorithm.
This section describes how to consume a number from a stream of code points.
It returns a 3-tuple of
a string representation,
a numeric value,
and a type flag 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 repr to the empty string
and type to "integer".
If the next input code point is U+002B PLUS SIGN (+) or U+002D HYPHEN-MINUS (-),
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:
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:
A sign:
a single U+002B PLUS SIGN (+) or U+002D HYPHEN-MINUS (-),
or the empty string.
Let s be the number -1 if the sign is U+002D HYPHEN-MINUS (-);
otherwise, let s be the number 1.
An integer part:
zero or more digits.
If there is at least one digit,
let i be the number formed by interpreting the digits as a base-10 integer;
otherwise, let i be the number 0.
A decimal point:
a single U+002E FULL STOP (.),
or the empty string.
A fractional part:
zero or more digits.
If there is at least one digit,
let f be the number formed by interpreting the digits as a base-10 integer
and d be the number of digits;
otherwise, let f and d 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 t be the number -1 if the sign is U+002D HYPHEN-MINUS (-);
otherwise, let t be the number 1.
An exponent:
zero or more digits.
If there is at least one digit,
let e be the number formed by interpreting the digits as a base-10 integer;
otherwise, let e be the number 0.
Return the number s·(i + f·10-d)·10te.
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 <bad-url-token> rather than a <url-token>.
It returns nothing;
its sole use is to consume enough of the input stream to reach a recovery point
where normal tokenizing can resume.
consume an escaped code point.
This allows an escaped right parenthesis ("\)") to be encountered without ending the <bad-url-token>.
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
A declaration has a name,
a value consisting of a list of component values,
and an important flag which is initially unset.
Should we go ahead and generalize the important flag to be a list of bang values?
Suggested by Zack Weinberg.
Declarations are further categorized as "properties" or "descriptors",
with the former typically appearing in qualified rules
and the latter appearing 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,
a function,
or a simple block.
preserved tokens
Any token produced by the tokenizer
except for <function-token>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>>, <bad-string-token>, and <bad-url-token> 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-grainted 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
A simple block has an associated token (either a <<[-token>>, <<(-token>>, or <<{-token>>)
and a value consisting of a list of component values.
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.
Railroad diagrams are more compact than a state-machine,
but often easier to read than a regular expression.
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.
When two strings are to be matched ASCII case-insensitively,
temporarily convert both of them to ASCII lower-case form
by adding 32 (0x20) to the value of each code point between
U+0041 LATIN CAPITAL LETTER A (A)
and U+005A LATIN CAPITAL LETTER Z (Z),
inclusive,
and check if this results in identical sequences of code point.
5.3
Parser Entry Points
The algorithms defined in this section produce high-level CSS objects
from lower-level objects.
They assume that they are invoked on a token stream,
but they may also be invoked on a string;
if so,
first perform input preprocessing
to produce a code point stream,
then perform tokenization
to produce a token stream.
"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 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.
All of the algorithms defined in this spec may be called with either a list of tokens or of component values.
Either way produces an identical result.
Consume a declaration.
If anything was returned, return it.
Otherwise, return a syntax error.
5.3.5
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 should be ignored by the consumer.
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 <EOF-token> 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 <EOF-token>s).
Otherwise,
it is implicitly invoked with the same list as the invoking algorithm.
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.
If the last two non-<whitespace-token>s in the declaration’s value are
a <delim-token> with the value "!"
followed by an <ident-token> 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.
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 (A) and offset (B),
and represents the An+Bth elements in a list,
for every positive integer or zero value of n,
with the first element in the list having index 1 (not 0).
For values of A and B greater than 0,
this effectively divides the list into groups of A elements
(the last group taking the remainder),
and selecting the Bth 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 A and B can be negative,
but only the positive results of An+B,
for n ≥ 0,
are used.
Example:
-n+6 /* represents the first 6 elements of the list */
If both A and B are 0,
the pseudo-class represents no element in the list.
6.1
Informal Syntax Description
This section is non-normative.
When A is 0, the An part may be omitted
(unless the B part is already omitted).
When An is not included
and B is non-negative,
the + sign before B (when allowed)
may also be omitted.
In this case the syntax simplifies to just B.
Examples:
0n+5 /* represents the 5th element in the list */
5 /* same */
When A is 1 or -1,
the 1 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 B is 0, then every Ath element is picked.
In such a case,
the +B (or -B) part may be omitted
unless the A part is already omitted.
Examples:
2n+0 /* represents every even element in the list */
2n /* same */
Whitespace is permitted on either side of the + or -
that separates the An and B 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 <an+b> 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 <an+b> type for CSS grammar purposes),
and how to interpret the CSS tokens to obtain values for A and B.
<ndashdigit-dimension> is a <dimension-token> 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
<integer> is a <number-token> with its type flag set to "integer"
<signed-integer> is a <number-token> with its type flag set to "integer", and whose representation starts with "+" or "-"
<signless-integer> is a <number-token> with its type flag set to "integer", and whose representation start 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.
The clauses of the production are interpreted as follows:
odd
A is 2, B is 1.
even
A is 2, B is 0.
<integer>
A is 0, B is the integer’s value.
<n-dimension>
'+'? n
-n
A is the dimension’s value, 1, or -1, respectively.
B is 0.
<ndashdigit-dimension>
'+'? <ndashdigit-ident>
A is the dimension’s value or 1, respectively.
B 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.
<dashndashdigit-ident>
A is -1.
B 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-dimension><signed-integer>
'+'? n <signed-integer>
-n <signed-integer>
A is the dimension’s value, 1, or -1, respectively.
B is the integer’s value.
<ndash-dimension><signless-integer>
'+'? n- <signless-integer>
-n- <signless-integer>
A is the dimension’s value, 1, or -1, respectively.
B is the negation of the integer’s value.
<n-dimension> ['+' | '-'] <signless-integer>
'+'? n ['+' | '-'] <signless-integer>
-n ['+' | '-'] <signless-integer>
A is the dimension’s value, 1, or -1, respectively.
B is the integer’s value.
If a '-' was provided between the two, B is instead the negation of the integer’s value.
7
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 <foo> refers to the "foo" grammar term,
assumed to be defined elsewhere.
Substituting the <foo> for its definition results in a semantically identical grammar.
Several types of tokens are written literally, without quotes:
<ident-token>s (such as auto, disc, etc), which are simply written as their value.
<at-keyword-token>s, which are written as an @ character followed by the token’s value, like "@media".
<function-token>s, which are written as the function name followed by a ( character, like "translate(".
Tokens match if their value is an ASCII case-insensitive match
for the value defined in the grammar.
Although it is possible, with escaping,
to construct an <ident-token> whose value starts with @ or ends with (,
such a tokens is not an <at-keyword-token> or a <function-token>
and does not match corresponding grammar definitions.
<delim-token>s are written with their value enclosed in single quotes.
For example, a <delim-token> 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.
<whitespace-token> is never indicated in the grammar;
<whitespace-token>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:
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.
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 <declaration-list> 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 <rule-list> 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 <stylesheet> production represents a list of rules.
It is identical to <rule-list>,
except that blocks using it default to accepting all rules
that aren’t otherwise limited to a particular context.
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:
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:
For rules that use <declaration-list>,
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 <declaration-list>,
!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 has its usual effect on the cascade origin of the property.
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 <rule-list>,
the spec for the rule must define what types of rules are valid inside the rule,
same as <declaration-list>,
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 <keyframe-rule>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 <stylesheet>,
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:
It additionally defines a restriction that the <stylesheet> can not contain @media rules,
which causes them to be dropped from the outer rule’s value if they appear.
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.
8.1
Style rules
A style rule is a qualified rule
that associates a selector list[SELECT]
with a list of property declarations.
They are also called
rule sets in [CSS21].
CSS Cascading and Inheritance [CSS3CASCADE] defines how the declarations inside of style rules participate in the cascade.
The prelude of the qualified rule is parsed as a
selector list.
If this results in an invalid selector list,
the entire style rule is invalid.
The content of the qualified rule’s block is parsed as a
list of declarations.
Unless defined otherwise by another specification or a future level of this specification,
at-rules in that list are invalid
and must be ignored.
Declaration 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 are not [CSS3-ANIMATIONS].
Note: However, the algorithm to determine the fallback encoding
looks at the first several bytes of the stylesheet
to see if they’re a match for the ASCII characters @charset "XXX";,
where XXX is a sequence of bytes other than 22 (ASCII for ").
While this resembles an @charset rule,
it’s not actually the same thing.
For example, the necessary sequence of bytes will spell out something entirely different
if the stylesheet is in an encoding that’s not ASCII-compatible,
such as UTF-16.
9
Serialization
This specification does not define how to serialize CSS in general,
leaving that task to the CSSOM and individual feature specifications.
However, there is one important facet that must be specified here regarding comments,
to ensure accurate "round-tripping" of data from text to CSS objects and back.
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,
but must be serialized in its position as
"/*"
followed by its contents
followed by "*/".
Unless the implementation has a preserved comment at that position,
it must insert the text "/**/" between the serialization of adjacent tokens
when the two tokens are of the following pairs:
Note: The preceding pairs of tokens can only be adjacent due to comments in the original text,
so the above rule reinserts the minimum number of comments into the serialized text to ensure an accurate round-trip.
(Roughly. The <delim-token> rules are slightly too powerful, for simplicity.)
Note: No comment is inserted between consecutive <whitespace-token>.
As a consequence such token sequences will not "round-trip" exactly.
This shouldn’t be an issue as CSS grammars always interpret
any amount of whitespace as identical to a single space.
9.1
Serializing <an+b>
To serialize an <an+b> value,
let s initially be the empty string:
A and B are both zero
Append "0" to s.
A is zero, B is non-zero
Serialize B and append it to s.
A is non-zero, B is zero
Serialize A and append it to s.
Append "n" to s.
A and B are both non-zero
Serialize A and append it to s.
Append "n" to s.
If B is positive,
append "+" to s
Serialize B and append it to s.
Return s.
10
Changes from CSS 2.1 and Selectors Level 3
This section is non-normative.
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 the the Encoding Standard
rather than the IANA registery 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.
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 <ident-token>s.
The <unicode-range-token> is now more restrictive,
and doesn’t include non-sensical patterns that the 2.1 definition allowed.
The BAD_URI token (now <bad-url-token>) is "self-contained".
In other words, once the tokenizer realizes it’s in a <bad-url-token> rather than a <url-token>,
it just seeks forward to look for the closing ),
ignoring everything else.
This behavior is simpler than treating it like a <function-token>
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.
<number-token>, <number-token>, and <dimension-token> have been changed
to include the preceding +/- sign as part of their value
(rather than as a separate <delim-token> 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.
<column-token> has been added,
to keep Selectors parsing in single-token lookahead.
Hexadecimal escape for surrogate code points now emit a replacement character rather than the surrogate.
This allows implementations to safely use UTF-16 internally.
Parsing changes:
Any list of declaration 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 and lets the next declaration being
at a {} block without a <semicolon-token>.
The handling of some miscellanous "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:
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 <dimension-token> or <ident-token>).
Acknowledgments
Thanks for feedback and contributions from
David Baron,
呂康豪 (Kang-Hao Lu),
Marc O’Morain,
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.
Conformance classes
Conformance to this specification
is defined for three conformance classes:
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.
Experimental implementations
To avoid clashes with future CSS features, the CSS2.1 specification
reserves a prefixed
syntax for proprietary and experimental extensions to CSS.
Prior to a specification reaching the Candidate Recommendation stage
in the W3C process, all implementations of a CSS feature are considered
experimental. The CSS Working Group recommends that implementations
use a vendor-prefixed syntax for such features, including those in
W3C Working Drafts. This avoids incompatibilities with future changes
in the draft.
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.
Anne says that steps 3/4 should be an input to this algorithm from the specs that define importing stylesheet,
to make the algorithm as a whole cleaner.
Perhaps abstract it into the concept of an "environment charset" or something?
↵
Should we only take the charset from the referring document if it’s same-origin?
↵
Should we go ahead and generalize the important flag to be a list of bang values?
Suggested by Zack Weinberg.
↵
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 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 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.