Cascading Style Sheets (CSS) Level 2

1. About the CSS 2.2 Specification

1.1. CSS 2.2 vs CSS 2

The CSS community has gained significant experience with the CSS2 specification since it became a recommendation in 1998. Errors in the CSS2 specification [CSS20] have subsequently been corrected in the first revised edition [CSS21] in 2011, but new errata were necessary.

While many of the issues will be addressed by the upcoming CSS3 specifications, the current state of affairs hinders the implementation and interoperability of CSS2. The CSS 2.2 specification attempts to address this situation by:

• Maintaining compatibility with those portions of CSS2 that are widely accepted and implemented.
• Incorporating all published CSS2 errata.
• Where implementations overwhelmingly differ from the CSS2 specification, modifying the specification to be in accordance with generally accepted practice.
• Removing CSS2 features which, by virtue of not having been implemented, have been rejected by the CSS community. CSS 2.2 aims to reflect what CSS features are reasonably widely implemented for HTML and XML languages in general (rather than only for a particular XML language, or only for HTML).
• Removing CSS2 features that will be obsoleted by CSS3, thus encouraging adoption of the proposed CSS3 features in their place.
• Adding a (very) small number of new property values, when implementation experience has shown that they are needed for implementing CSS2.

Thus, while it is not the case that a CSS2 style sheet is necessarily forwards-compatible with CSS 2.2, it is the case that a style sheet restricting itself to CSS 2.2 features is more likely to find a compliant user agent today and to preserve forwards compatibility in the future. While breaking forward compatibility is not desirable, we believe the advantages to the revisions in CSS 2.2 are worthwhile.

CSS 2.2 is derived from and is intended to replace CSS 2.1 and CSS2. Some parts of CSS2 are unchanged in CSS 2.2, some parts have been altered, and some parts removed. The removed portions may be used in a future CSS3 specification. Future specs should refer to CSS 2.2 (unless they need features from CSS2 which have been dropped in CSS 2.2, and then they should only reference CSS2 for those features, or preferably reference such feature(s) in the respective CSS3 Module that includes those feature(s)).

1.4. Conventions

1.4.1. Document language elements and attributes

• CSS property and pseudo-class names are delimited by single quotes.
• CSS values are delimited by single quotes.
• Document language attribute names are in lowercase letters and delimited by double quotes.

1.4.2. CSS property definitions

Each CSS property definition begins with a summary of key information that resembles the following:

1.4.2.1. Value

This part specifies the set of valid values for the property whose name is property-name. A property value can have one or more components. Component value types are designated in several ways:

1. keyword values (e.g., auto, disc, etc.)
2. basic data types, which appear between "<" and ">" (e.g., <length>, <percentage>, etc.). In the electronic version of the document, each instance of a basic data type links to its definition.
3. types that have the same range of values as a property bearing the same name (e.g., <border-width> <background-attachment>, etc.). In this case, the type name is the property name (complete with quotes) between "<" and ">" (e.g., <border-width>). Such a type does not include the value inherit. In the electronic version of the document, each instance of this type of non-terminal links to the corresponding property definition.
4. non-terminals that do not share the same name as a property. In this case, the non-terminal name appears between "<" and ">", as in <border-width>. Notice the distinction between <border-width> and <border-width>; the latter is defined in terms of the former. The definition of a non-terminal is located near its first appearance in the specification. In the electronic version of the document, each instance of this type of value links to the corresponding value definition.

Other words in these definitions are keywords that must appear literally, without quotes (e.g., red). The slash (/) and the comma (,) must also appear literally.

Component values may be arranged into property values as follows:

• Several juxtaposed words mean that all of them must occur, in the given order.
• A bar (|) separates two or more alternatives: exactly one of them must occur.
• A double bar (||) separates two or more options: one or more of them must occur, in any order.
• A double ampersand (&&) separates two or more components, all of which must occur, in any order.
• Brackets ([ ]) are for grouping.

Juxtaposition is stronger than the double ampersand, the double ampersand is stronger than the double bar, and the double bar is stronger than the bar. Thus, the following lines are equivalent:

  a b   |   c ||   d &&   e f
[ a b ] | [ c || [ d && [ e f ]]]

Every type, keyword, or bracketed group may be followed by one of the following modifiers:

• An asterisk (*) indicates that the preceding type, word, or group occurs zero or more times.
• A plus (+) indicates that the preceding type, word, or group occurs one or more times.
• A question mark (?) indicates that the preceding type, word, or group is optional.
• A pair of numbers in curly braces ({A,B}) indicates that the preceding type, word, or group occurs at least A and at most B times.

The following examples illustrate different value types:

Value: N | NW | NE
Value: [ <length> | thick | thin ]{1,4}
Value: [<family-name> , ]* <family-name>
Value: <uri>? <color> [ / <color> ]?
Value: <uri> || <color>
Value: inset? && [ <length>{2,4} && <color>? ]

Component values are specified in terms of tokens, as described in Appendix G.2. As the grammar allows spaces between tokens in the components of the expr production, spaces may appear between tokens in property values.

Note: In many cases, spaces will in fact be required between tokens in order to distinguish them from each other. For example, the value 1em2em would be parsed as a single DIMEN token with the number 1 and the identifier em2em, which is an invalid unit. In this case, a space would be required before the 2 to get this parsed as the two lengths 1em and 2em.

1.4.2.2. Initial

This part specifies the property’s initial value. Please consult the section on the cascade for information about the interaction between style sheet-specified, inherited, and initial property values.

1.4.2.3. Applies to

This part lists the elements to which the property applies. All elements are considered to have all properties, but some properties have no rendering effect on some types of elements. For example, the clear property only affects block-level elements.

1.4.2.4. Inherited

This part indicates whether the value of the property is inherited from an ancestor element. Please consult the section on the cascade for information about the interaction between style sheet-specified, inherited, and initial property values.

1.4.2.5. Percentage values

This part indicates how percentages should be interpreted, if they occur in the value of the property. If "N/A" appears here, it means that the property does not accept percentages in its values.

1.4.2.6. Media groups

This part indicates the media groups to which the property applies. Information about media groups is non-normative.

1.4.2.7. Computed value

This part describes the computed value for the property. See the section on computed values for how this definition is used.

1.4.3. Shorthand properties

Some properties are shorthand properties, meaning that they allow authors to specify the values of several properties with a single property.

For instance, the font property is a shorthand property for setting font-style, font-variant, font-weight, font-size, line-height, and font-family all at once.

When values are omitted from a shorthand form, each "missing" property is assigned its initial value (see the section on the cascade).

h1 {
  font-weight: bold;
  font-size: 12pt;
  line-height: 14pt;
  font-family: Helvetica;
  font-variant: normal;
  font-style: normal;
}

h1 { font: bold 12pt/14pt Helvetica }

1.4.4. Notes and examples

All examples that illustrate illegal usage are clearly marked as "ILLEGAL EXAMPLE".

HTML examples lacking DOCTYPE declarations are SGML Text Entities conforming to the HTML 4.01 Strict DTD [HTML401]. Other HTML examples conform to the DTDs given in the examples.

All notes are informative only.

Examples and notes are marked within the source HTML for the specification and CSS user agents will render them specially.

1.4.5. Images and long descriptions

Most images in the electronic version of this specification are accompanied by "long descriptions" of what they represent. A link to the long description is denoted by a "[D]" after the image.

Images and long descriptions are informative only.

2. Introduction to CSS 2

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
  <HEAD>
  <TITLE>Bach’s home page</TITLE>
  </HEAD>
  <BODY>
    <H1>Bach’s home page</H1>
    <P>Johann Sebastian Bach was a prolific composer.
  </BODY>
</HTML>

h1 { color: red }

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
  <HEAD>
  <TITLE>Bach’s home page</TITLE>
  <STYLE type="text/css">
    h1 { color: red }
  </STYLE>
  </HEAD>
  <BODY>
    <H1>Bach’s home page</H1>
    <P>Johann Sebastian Bach was a prolific composer.
  </BODY>
</HTML>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
  <HEAD>
  <TITLE>Bach’s home page</TITLE>
  <LINK rel="stylesheet" href="bach.css" type="text/css">
  </HEAD>
  <BODY>
    <H1>Bach’s home page</H1>
    <P>Johann Sebastian Bach was a prolific composer.
  </BODY>
</HTML>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
  <HEAD>
  <TITLE>Bach’s home page</TITLE>
  <STYLE type="text/css">
    body { color: black; background: white }
    h1 { color: red; background: white }
  </STYLE>
  </HEAD>
  <BODY>
    <H1>Bach’s home page</H1>
    <P>Johann Sebastian Bach was a prolific composer.
  </BODY>
</HTML>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
  <HEAD>
  <TITLE>Bach’s home page</TITLE>
  <STYLE type="text/css">
    body {
      font-family: "Gill Sans", sans-serif;
      font-size: 12pt;
      margin: 3em;
    }
  </STYLE>
  </HEAD>
  <BODY>
    <H1>Bach’s home page</H1>
    <P>Johann Sebastian Bach was a prolific composer.
  </BODY>
</HTML>

<ARTICLE>
  <HEADLINE>Fredrick the Great meets Bach</HEADLINE>
  <AUTHOR>Johann Nikolaus Forkel</AUTHOR>
  <PARA>
    One evening, just as he was getting his
    <INSTRUMENT>flute</INSTRUMENT> ready and his
    musicians were assembled, an officer brought him a list of
    the strangers who had arrived.
  </PARA>
</ARTICLE>

INSTRUMENT { display: inline }
ARTICLE, HEADLINE, AUTHOR, PARA { display: block }

<?xml-stylesheet type="text/css" href="bach.css"?>
<ARTICLE>
  <HEADLINE>Fredrick the Great meets Bach</HEADLINE>
  <AUTHOR>Johann Nikolaus Forkel</AUTHOR>
  <PARA>
    One evening, just as he was getting his
    <INSTRUMENT>flute</INSTRUMENT> ready and his
    musicians were assembled, an officer brought him a list of
    the strangers who had arrived.
  </PARA>
</ARTICLE>

INSTRUMENT { display: inline }
ARTICLE, HEADLINE, AUTHOR, PARA { display: block }
HEADLINE { font-size: 1.3em }
AUTHOR { font-style: italic }
ARTICLE, HEADLINE, AUTHOR, PARA { margin: 0.5em }

2.3. The CSS 2 processing model

This section up to but not including its subsections is non-normative.

2.3.1. The canvas

For all media, the term canvas describes "the space where the formatting structure is rendered." The canvas is infinite for each dimension of the space, but rendering generally occurs within a finite region of the canvas, established by the user agent according to the target medium. For instance, user agents rendering to a screen generally impose a minimum width and choose an initial width based on the dimensions of the viewport. User agents rendering to a page generally impose width and height constraints. Aural user agents may impose limits in audio space, but not in time.

2.3.2. CSS 2 addressing model

CSS 2 selectors and properties allow style sheets to refer to the following parts of a document or user agent:

• Elements in the document tree and certain relationships between them (see the section on selectors).
• Attributes of elements in the document tree, and values of those attributes (see the section on attribute selectors).
• Some parts of element content (see the :first-line and :first-letter pseudo-elements).
• Elements of the document tree when they are in a certain state (see the section on pseudo-classes).
• Some aspects of the canvas where the document will be rendered.
• Some system information (see the section on user interface).

3. Conformance: Requirements and Recommendations

3.1. Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 (see [RFC2119]). However, for readability, these words do not appear in all uppercase letters in this specification.

At times, this specification recommends good practice for authors and user agents. These recommendations are not normative and conformance with this specification does not depend on their realization. These recommendations contain the expression "We recommend ...", "This specification recommends ...", or some similar wording.

The fact that a feature is marked as deprecated or going to be deprecated in CSS3 (namely the system colors) also has no influence on conformance. (For example, the system colors are a normative part of the specification, so UAs must support them.)

All sections of this specification, including appendices, are normative unless otherwise noted.

Examples and notes are not normative.

Figures are for illustration only. They are not reference renderings, unless explicitly stated.

Style sheet

A set of statements that specify presentation of a document.

Style sheets may have three different origins: author, user, and user agent. The interaction of these sources is described in the section on cascading and inheritance.

Valid style sheet

The validity of a style sheet depends on the level of CSS used for the style sheet. All valid CSS1 style sheets are valid CSS 2 style sheets, but some changes from CSS1 mean that a few CSS1 style sheets will have slightly different semantics in CSS 2. Some features in CSS2 (1998) are not part of CSS 2, so not all CSS2 (1998) style sheets are valid CSS 2 style sheets.

A valid CSS 2 style sheet must be written according to the grammar of CSS 2. Furthermore, it must contain only at-rules, property names, and property values defined in this specification. An illegal (invalid) at-rule, property name, or property value is one that is not valid.

Source document

The document to which one or more style sheets apply. This is encoded in some language that represents the document as a tree of elements. Each element consists of a name that identifies the type of element, optionally a number of attributes, and a (possibly empty) content. For example, the source document could be an XML or SGML instance.

Document language

The encoding language of the source document (e.g., HTML, XHTML, or SVG). CSS is used to describe the presentation of document languages and CSS does not change the underlying semantics of the document languages.

Element

(An SGML term, see [ISO8879].) The primary syntactic constructs of the document language. Most CSS style sheet rules use the names of these elements (such as P, TABLE, and OL in HTML) to specify how the elements should be rendered.

Replaced element

An element whose content is outside the scope of the CSS formatting model, such as an image, embedded document, or applet. For example, the content of the HTML IMG element is often replaced by the image that its "src" attribute designates. Replaced elements often have intrinsic dimensions: an intrinsic width, an intrinsic height, and an intrinsic ratio. For example, a bitmap image has an intrinsic width and an intrinsic height specified in absolute units (from which the intrinsic ratio can obviously be determined). On the other hand, other documents may not have any intrinsic dimensions (for example, a blank HTML document).

User agents may consider a replaced element to not have any intrinsic dimensions if it is believed that those dimensions could leak sensitive information to a third party. For example, if an HTML document changed intrinsic size depending on the user’s bank balance, then the UA might want to act as if that resource had no intrinsic dimensions.

The content of replaced elements is not considered in the CSS rendering model.

Intrinsic dimensions

The width and height as defined by the element itself, not imposed by the surroundings. CSS does not define how the intrinsic dimensions are found. In CSS 2 only replaced elements can come with intrinsic dimensions. For raster images without reliable resolution information, a size of 1 px unit per image source pixel must be assumed.

Attribute

A value associated with an element, consisting of a name, and an associated (textual) value.

Content

The content associated with an element in the source document. Some elements have no content, in which case they are called empty. The content of an element may include text, and it may include a number of sub-elements, in which case the element is called the parent of those sub-elements.

Ignore

This term has two slightly different meanings in this specification. First, a CSS parser must follow certain rules when it discovers unknown or illegal syntax in a style sheet. The parser must then ignore certain parts of the style sheets. The exact rules for which parts must be ignored are described in these sections (Declarations and properties, Rules for handling parsing errors, Unsupported Values) or may be explained in the text where the term "ignore" appears. Second, a user agent may (and, in some cases must) disregard certain properties or values in the style sheet, even if the syntax is legal. For example, table-column elements cannot affect the font of the column, so the font properties must be ignored.

Rendered content

The content of an element after the rendering that applies to it according to the relevant style sheets has been applied. How a replaced element’s content is rendered is not defined by this specification. Rendered content may also be alternate text for an element (e.g., the value of the XHTML "alt" attribute), and may include items inserted implicitly or explicitly by the style sheet, such as bullets, numbering, etc.

Document tree

The tree of elements encoded in the source document. Each element in this tree has exactly one parent, with the exception of the root element, which has none.

Child

An element A is called the child of element B if and only if B is the parent of A.

Descendant

An element A is called a descendant of an element B, if either (1) A is a child of B, or (2) A is the child of some element C that is a descendant of B.

Ancestor

An element A is called an ancestor of an element B, if and only if B is a descendant of A.

Sibling

An element A is called a sibling of an element B, if and only if B and A share the same parent element. Element A is a preceding sibling if it comes before B in the document tree. Element B is a following sibling if it comes after A in the document tree.

Preceding element

An element A is called a preceding element of an element B, if and only if (1) A is an ancestor of B or (2) A is a preceding sibling of B.

Following element

An element A is called a following element of an element B, if and only if B is a preceding element of A.

Author

An author is a person who writes documents and associated style sheets. An authoring tool is a user agent that generates style sheets.

User

A user is a person who interacts with a user agent to view, hear, or otherwise use a document and its associated style sheet. The user may provide a personal style sheet that encodes personal preferences.

User agent (UA)

A user agent is any program that interprets a document written in the document language and applies associated style sheets according to the terms of this specification. A user agent may display a document, read it aloud, cause it to be printed, convert it to another format, etc.

An HTML user agent is one that supports one or more of the HTML specifications. A user agent that supports XHTML [XHTML1], but not HTML is not considered an HTML user agent for the purpose of conformance with this specification.

Property

CSS defines a finite set of parameters, called properties, that direct the rendering of a document. Each property has a name (e.g., color, font, or border') and a value (e.g., red, '12pt Times', or dotted). Properties are attached to various parts of the document and to the page on which the document is to be displayed by the mechanisms of specificity, cascading, and inheritance (see the chapter on Assigning property values, Cascading, and Inheritance).

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
  <TITLE>My home page</TITLE>
  <BODY>
    <H1>My home page</H1>
    <P>Welcome to my home page! Let me tell you about my favorite
    composers:
    <UL>
      <LI> Elvis Costello
      <LI> Johannes Brahms
      <LI> Georges Brassens
    </UL>
  </BODY>
</HTML>

3.2. UA Conformance

This section defines conformance with the CSS 2 specification only. There may be other levels of CSS in the future that may require a user agent to implement a different set of features in order to conform.

In general, the following points must be observed by a user agent claiming conformance to this specification:

1. It must recognize one or more of the CSS 2 media types.
2. For each source document, it must attempt to retrieve all associated style sheets that are appropriate for the recognized media types. If it cannot retrieve all associated style sheets (for instance, because of network errors), it must display the document using those it can retrieve.
3. It must parse the style sheets according to this specification. In particular, it must recognize all at-rules, blocks, declarations, and selectors (see the grammar of CSS 2). If a user agent encounters a property that applies for a supported media type, the user agent must parse the value according to the property definition. This means that the user agent must accept all valid values and must ignore declarations with invalid values. User agents must ignore rules that apply to unsupported media types.
4. For each element in a document tree, it must assign a value for every property according to the property’s definition and the rules of cascading and inheritance.
5. If the source document comes with alternate style sheet sets (such as with the "alternate" keyword in HTML 4 [HTML401]), the UA must allow the user to select which style sheet set the UA should apply.
6. The UA must allow the user to turn off the influence of author style sheets.

Not every user agent must observe every point, however:

• An application that reads style sheets without rendering any content (e.g., a CSS 2 validator) must respect points 1-3.
• An authoring tool is only required to output valid style sheets
• A user agent that renders a document with associated style sheets must respect points 1-6 and render the document according to the media-specific requirements set forth in this specification. Values may be approximated when required by the user agent.

The inability of a user agent to implement part of this specification due to the limitations of a particular device (e.g., a user agent cannot render colors on a monochrome monitor or page) does not imply non-conformance.

UAs must allow users to specify a file that contains the user style sheet. UAs that run on devices without any means of writing or specifying files are exempted from this requirement. Additionally, UAs may offer other means to specify user preferences, for example, through a GUI.

CSS 2 does not define which properties apply to form controls and frames, or how CSS can be used to style them. User agents may apply CSS properties to these elements. Authors are recommended to treat such support as experimental. A future level of CSS may specify this further.

3.3. Error conditions

In general, this document specifies error handling behavior throughout the specification. For example, see the rules for handling parsing errors.

3.4. The text/css content type

The media type (commonly MIME type) of text/css has been registered by [RFC2318].

4. Syntax and basic data types

4.1. Syntax

This section describes a grammar (and forward-compatible parsing rules) common to any level of CSS (including CSS 2). Future updates of CSS will adhere to this core syntax, although they may add additional syntactic constraints.

These descriptions are normative. They are also complemented by the normative grammar rules presented in Appendix G.

In this specification, the expressions "immediately before" or "immediately after" mean with no intervening white space or comments.

4.1.1. Tokenization

All levels of CSS — level 1, level 2, and any future levels — use the same core syntax. This allows UAs to parse (though not completely understand) style sheets written in levels of CSS that did not exist at the time the UAs were created. Designers can use this feature to create style sheets that work with older user agents, while also exercising the possibilities of the latest levels of CSS.

At the lexical level, CSS style sheets consist of a sequence of tokens. The list of tokens for CSS is as follows. The definitions use Lex-style regular expressions. Octal codes refer to ISO 10646 ([ISO10646]). As in Lex, in case of multiple matches, the longest match determines the token.

The macros in curly braces ({}) above are defined as follows:

Below is the core syntax for CSS. The sections that follow describe how to use it. Appendix G describes a more restrictive grammar that is closer to the CSS level 2 language. Parts of style sheets that can be parsed according to this grammar but not according to the grammar in Appendix G are among the parts that will be ignored according to the rules for handling parsing errors.

stylesheet  : [ CDO | CDC | S | statement ]*;
statement   : ruleset | at-rule;
at-rule     : ATKEYWORD S* any* [ block | ';' S* ];
block       : '{' S* [ any | block | ATKEYWORD S* | ';' S* ]* '}' S*;
ruleset     : selector? '{' S* declaration? [ ';' S* declaration? ]* '}' S*;
selector    : any+;
declaration : property S* ':' S* value;
property    : IDENT;
value       : [ any | block | ATKEYWORD S* ]+;
any         : [ IDENT | NUMBER | PERCENTAGE | DIMENSION | STRING
              | DELIM | URI | HASH | UNICODE-RANGE | INCLUDES
              | DASHMATCH | ':' | FUNCTION S* [any|unused]* ')'
              | '(' S* [any|unused]* ')' | '[' S* [any|unused]* ']'
              ] S*;
unused      : block | ATKEYWORD S* | ';' S* | CDO S* | CDC S*;

The "unused" production is not used in CSS and will not be used by any future extension. It is included here only to help with error handling. (See 4.2 "Rules for handling parsing errors.")

COMMENT tokens do not occur in the grammar (to keep it readable), but any number of these tokens may appear anywhere outside other tokens. (Note, however, that a comment before or within the @charset rule disables the @charset.)

The token S in the grammar above stands for white space. Only the characters "space" (U+0020), "tab" (U+0009), "line feed" (U+000A), "carriage return" (U+000D), and "form feed" (U+000C) can occur in white space. Other space-like characters, such as "em-space" (U+2003) and "ideographic space" (U+3000), are never part of white space.

The meaning of input that cannot be tokenized or parsed is undefined in CSS 2.

4.1.2. Keywords

Keywords have the form of identifiers. Keywords must not be placed between quotes ("..." or ...). Thus,

red

is a keyword, but

"red"

is not. (It is a string.) Other illegal examples:

width: "auto";
border: "none";
background: "red";

4.1.2.1. Vendor-specific extensions

In CSS, identifiers may begin with '-' (dash) or '_' (underscore). Keywords and property names beginning with -' or '_' are reserved for vendor-specific extensions. Such vendor-specific extensions should have one of the following formats:

'-' + vendor identifier + '-' + meaningful name
'_' + vendor identifier + '-' + meaningful name

-moz-box-sizing
-moz-border-radius
-wap-accesskey

An initial dash or underscore is guaranteed never to be used in a property or keyword by any current or future level of CSS. Thus typical CSS implementations may not recognize such properties and may ignore them according to the rules for handling parsing errors. However, because the initial dash or underscore is part of the grammar, CSS 2 implementers should always be able to use a CSS-conforming parser, whether or not they support any vendor-specific extensions.

Authors should avoid vendor-specific extensions

4.1.3. Characters and case

The following rules always hold:

• All CSS syntax is case-insensitive within the ASCII range (i.e., [a-z] and [A-Z] are equivalent), except for parts that are not under the control of CSS. For example, the case-sensitivity of values of the HTML attributes "id" and "class", of font names, and of URIs lies outside the scope of this specification. Note in particular that element names are case-insensitive in HTML, but case-sensitive in XML.
• In CSS, identifiers (including element names, classes, and IDs in selectors) can contain only the characters [a-zA-Z0-9] and ISO 10646 characters U+00A0 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit, two hyphens, or a hyphen followed by a digit. Identifiers can also contain escaped characters and any ISO 10646 character as a numeric code (see next item). For instance, the identifier "B&W?" may be written as "B\&W\?" or "B\26 W\3F".

  Note that Unicode is code-by-code equivalent to ISO 10646 (see [UNICODE] and [ISO10646]).

• In CSS 2, a backslash (\) character can indicate one of three types of character escape. Inside a CSS comment, a backslash stands for itself, and if a backslash is immediately followed by the end of the style sheet, it also stands for itself (i.e., a DELIM token).

  First, inside a string, a backslash followed by a newline is ignored (i.e., the string is deemed not to contain either the backslash or the newline). Outside a string, a backslash followed by a newline stands for itself (i.e., a DELIM followed by a newline).

  Second, it cancels the meaning of special CSS characters. Any character (except a hexadecimal digit, linefeed, carriage return, or form feed) can be escaped with a backslash to remove its special meaning. For example, "\"" is a string consisting of one double quote. Style sheet preprocessors must not remove these backslashes from a style sheet since that would change the style sheet’s meaning.

  Third, backslash escapes allow authors to refer to characters they cannot easily put in a document. In this case, the backslash is followed by at most six hexadecimal digits (0..9A..F), which stand for the ISO 10646 ([ISO10646]) character with that number, which must not be zero. (It is undefined in CSS 2 what happens if a style sheet does contain a character with Unicode codepoint zero.) If a character in the range [0-9a-fA-F] follows the hexadecimal number, the end of the number needs to be made clear. There are two ways to do that:

  1. with a space (or other white space character): "\26 B" ("&B"). In this case, user agents should treat a "CR/LF" pair (U+000D/U+000A) as a single white space character.
  2. by providing exactly 6 hexadecimal digits: "\000026B" ("&B")

  In fact, these two methods may be combined. Only one white space character is ignored after a hexadecimal escape. Note that this means that a "real" space after the escape sequence must be doubled.

  If the number is outside the range allowed by Unicode (e.g., "\110000" is above the maximum 10FFFF allowed in current Unicode), the UA may replace the escape with the "replacement character" (U+FFFD). If the character is to be displayed, the UA should show a visible symbol, such as a "missing character" glyph (cf. 15.2, point 5).

• Note: Backslash escapes are always considered to be part of an identifier or a string (i.e., "\7B" is not punctuation, even though "{" is, and "\32" is allowed at the start of a class name, even though "2" is not).

  The identifier "te\st" is exactly the same identifier as "test".

4.1.4. Statements

A CSS style sheet, for any level of CSS, consists of a list of statements (see the grammar above). There are two kinds of statements: at-rules and rule sets. There may be white space around the statements.

4.1.5. At-rules

At-rules start with an at-keyword, an @ character followed immediately by an identifier (for example, @import, @page).

An at-rule consists of everything up to and including the next semicolon (;) or the next block, whichever comes first.

CSS 2 user agents must ignore any @import rule that occurs inside a block or after any non-ignored statement other than an @charset or an @import rule.

@import "subs.css";
h1 { color: blue }
@import "list.css";

@import "subs.css";
h1 { color: blue }

@import "subs.css";
@media print {
  @import "print-main.css";
  body { font-size: 10pt }
}
h1 {color: blue }

@import "subs.css";
@import "print-main.css" print;
@media print {
  body { font-size: 10pt }
}
h1 {color: blue }

4.1.6. Blocks

A block starts with a left curly brace ({) and ends with the matching right curly brace (}). In between there may be any tokens, except that parentheses (( )), brackets ([ ]), and braces ({ }) must always occur in matching pairs and may be nested. Single (') and double quotes (") must also occur in matching pairs, and characters between them are parsed as a string. See Tokenization above for the definition of a string.

{ causta: "}" + ({7} * '\'') }

4.1.7. Rule sets, declaration blocks, and selectors

A rule set (also called "rule") consists of a selector followed by a declaration block.

A declaration block starts with a left curly brace ({) and ends with the matching right curly brace (}). In between there must be a list of zero or more semicolon-separated (;) declarations.

The selector consists of everything up to (but not including) the first left curly brace ({). A selector always goes together with a declaration block. When a user agent cannot parse the selector (i.e., it is not valid CSS 2), it must ignore the selector and the following declaration block (if any) as well.

CSS 2 gives a special meaning to the comma (,) in selectors. However, since it is not known if the comma may acquire other meanings in future updates of CSS, the whole statement should be ignored if there is an error anywhere in the selector, even though the rest of the selector may look reasonable in CSS 2.

h1, h2 {color: green }
h3, h4 & h5 {color: red }
h6 {color: black }

p[example="public class foo\
{\
    private int x;\
\
    foo(int x) {\
        this.x = x;\
    }\
\
}"] { color: red }

4.1.8. Declarations and properties

A declaration is either empty or consists of a property name, followed by a colon (:), followed by a property value. Around each of these there may be white space.

Because of the way selectors work, multiple declarations for the same selector may be organized into semicolon (;) separated groups.

h1 { font-weight: bold }
h1 { font-size: 12px }
h1 { line-height: 14px }
h1 { font-family: Helvetica }
h1 { font-variant: normal }
h1 { font-style: normal }

h1 {
  font-weight: bold;
  font-size: 12px;
  line-height: 14px;
  font-family: Helvetica;
  font-variant: normal;
  font-style: normal
}

A property name is an identifier. Any token may occur in the property value. Parentheses ("( )"), brackets ("[ ]"), braces ("{ }"), single quotes ('), and double quotes (") must come in matching pairs, and semicolons not in strings must be escaped. Parentheses, brackets, and braces may be nested. Inside the quotes, characters are parsed as a string.

The syntax of values is specified separately for each property, but in any case, values are built from identifiers, strings, numbers, lengths, percentages, URIs, colors, etc.

A user agent must ignore a declaration with an invalid property name or an invalid value. Every CSS property has its own syntactic and semantic restrictions on the values it accepts.

h1 { color: red; font-style: 12pt }  /* Invalid value: 12pt */
p { color: blue;  font-vendor: any;  /* Invalid prop.: font-vendor */
    font-variant: small-caps }
em em { font-style: normal }

h1 { color: red; }
p { color: blue;  font-variant: small-caps }
em em { font-style: normal }

4.1.9. Comments

Comments begin with the characters "/*" and end with the characters "*/". They may occur anywhere outside other tokens, and their contents have no influence on the rendering. Comments may not be nested.

CSS also allows the SGML comment delimiters ("<!--" and "-->") in certain places defined by the grammar, but they do not delimit CSS comments. They are permitted so that style rules appearing in an HTML source document (in the STYLE element) may be hidden from pre-HTML 3.2 user agents. See the HTML 4 specification ([HTML401]) for more information.

4.2. Rules for handling parsing errors

In some cases, user agents must ignore part of an illegal style sheet. This specification defines ignore to mean that the user agent parses the illegal part (in order to find its beginning and end), but otherwise acts as if it had not been there. CSS 2 reserves for future updates of CSS all property:value combinations and @-keywords that do not contain an identifier beginning with dash or underscore. Implementations must ignore such combinations (other than those introduced by future updates of CSS).

To ensure that new properties and new values for existing properties can be added in the future, user agents are required to obey the following rules when they encounter the following scenarios:

• Unknown properties. User agents must ignore a declaration with an unknown property. For example, if the style sheet is:

  h1 { color: red; rotation: 70minutes }
  

  the user agent will treat this as if the style sheet had been

  h1 { color: red }
  

• Illegal values. User agents must ignore a declaration with an illegal value. For example:

  img { float: left }       /* correct CSS 2 */
  img { float: left here }  /* "here" is not a value of 'float' */
  img { background: "red" } /* keywords cannot be quoted */
  img { border-width: 3 }   /* a unit must be specified for length values */
  

  A CSS 2 parser would honor the first rule and ignore the rest, as if the style sheet had been:

  img { float: left }
  img { }
  img { }
  img { }
  

  A user agent conforming to a future CSS specification may accept one or more of the other rules as well.

• Malformed declarations. User agents must handle unexpected tokens encountered while parsing a declaration by reading until the end of the declaration, while observing the rules for matching pairs of (), [], {}, "", and '', and correctly handling escapes. For example, a malformed declaration may be missing a property name, colon (:), or property value. The following are all equivalent:

  p { color:green }
  p { color:green; color }  /* malformed declaration missing ':', value */
  p { color:red;   color; color:green }  /* same with expected recovery */
  p { color:green; color: } /* malformed declaration missing value */
  p { color:red;   color:; color:green } /* same with expected recovery */
  p { color:green; color{;color:maroon} } /* unexpected tokens { } */
  p { color:red;   color{;color:maroon}; color:green } /* same with recovery */
  

• Malformed statements. User agents must handle unexpected tokens encountered while parsing a statement by reading until the end of the statement, while observing the rules for matching pairs of (), [], {}, "", and '', and correctly handling escapes. For example, a malformed statement may contain an unexpected closing brace or at-keyword. E.g., the following lines are all ignored:

  p @here {color: red}     /* ruleset with unexpected at-keyword "@here" */
  @foo @bar;               /* at-rule with unexpected at-keyword "@bar" */
  }} {{ - }}               /* ruleset with unexpected right brace */
  ) ( {} ) p {color: red } /* ruleset with unexpected right parenthesis */
  

• At-rules with unknown at-keywords. User agents must ignore an invalid at-keyword together with everything following it, up to the end of the block that contains the invalid at-keyword, or up to and including the next semicolon (;), or up to and including the next block ({...}), whichever comes first. For example, consider the following:

  @three-dee {
    @background-lighting {
      azimuth: 30deg;
      elevation: 190deg;
    }
    h1 { color: red }
  }
  h1 { color: blue }
  

  The @three-dee at-rule is not part of CSS 2. Therefore, the whole at-rule (up to, and including, the third right curly brace) is ignored. A CSS 2 user agent ignores it, effectively reducing the style sheet to:

  h1 { color: blue }
  

  Something inside an at-rule that is ignored because it is invalid, such as an invalid declaration within an @media-rule, does not make the entire at-rule invalid.

• Unexpected end of style sheet.

  User agents must close all open constructs (for example: blocks, parentheses, brackets, rules, strings, and comments) at the end of the style sheet. For example:

  @media screen {
    p:before { content: 'Hello
  

  would be treated the same as:

  @media screen {
    p:before { content: 'Hello'; }
  }
  

  in a conformant UA.

• Unexpected end of string.

  User agents must close strings upon reaching the end of a line (i.e., before an unescaped line feed, carriage return or form feed character), but then drop the construct (declaration or rule) in which the string was found. For example:

  p {
    color: green;
    font-family: 'Courier New Times
    color: red;
    color: green;
  }
  

  ...would be treated the same as:

  p { color: green; color: green; }
  

  ...because the second declaration (from font-family to the semicolon after 'color: red') is invalid and is dropped.

• See also Rule sets, declaration blocks, and selectors for parsing rules for declaration blocks.

4.3. Values

4.3.1. Integers and real numbers

Some value types may have integer values (denoted by <integer>) or real number values (denoted by <number>). Real numbers and integers are specified in decimal notation only. An <integer> consists of one or more digits "0" to "9". A <number> can either be an <integer>, or it can be zero or more digits followed by a dot (.) followed by one or more digits. Both integers and real numbers may be preceded by a "-" or "+" to indicate the sign. \-0 is equivalent to 0 and is not a negative number.

Note that many properties that allow an integer or real number as a value actually restrict the value to some range, often to a non-negative value.

4.3.2. Lengths

Lengths refer to distance measurements.

The format of a length value (denoted by <length> in this specification) is a <number> (with or without a decimal point) immediately followed by a unit identifier (e.g., px, em, etc.). After a zero length, the unit identifier is optional.

Some properties allow negative length values, but this may complicate the formatting model and there may be implementation-specific limits. If a negative length value cannot be supported, it should be converted to the nearest value that can be supported.

If a negative length value is set on a property that does not allow negative length values, the declaration is ignored.

In cases where the used length cannot be supported, user agents must approximate it in the actual value.

There are two types of length units: relative and absolute. Relative length units specify a length relative

to another length property. Style sheets that use relative units can more easily scale from one output environment to another.

Relative units are:

• em: the font-size of the relevant font
• ex: the x-height of the relevant font

h1 { margin: 0.5em }      /* em */
h1 { margin: 1ex }        /* ex */

The em unit is equal to the computed value of the font-size property of the element on which it is used. The exception is when em occurs in the value of the font-size property itself, in which case it refers to the font size of the parent element. It may be used for vertical or horizontal measurement. (This unit is also sometimes called the quad-width in typographic texts.)

The ex unit is defined by the element’s first available font. The exception is when ex occurs in the value of the font-size property, in which case it refers to the ex of the parent element.

The x-height is so called because it is often equal to the height of the lowercase "x". However, an ex is defined even for fonts that do not contain an "x".

The x-height of a font can be found in different ways. Some fonts contain reliable metrics for the x-height. If reliable font metrics are not available, UAs may determine the x-height from the height of a lowercase glyph. One possible heuristic is to look at how far the glyph for the lowercase "o" extends below the baseline, and subtract that value from the top of its bounding box. In the cases where it is impossible or impractical to determine the x-height, a value of 0.5em should be used.

h1 { line-height: 1.2em }

h1 { font-size: 1.2em }

When specified for the root of the document tree (e.g., "HTML" in HTML), em and ex refer to the property’s initial value.

Child elements do not inherit the relative values specified for their parent; they inherit the computed values.

body {
  font-size: 12px;
  text-indent: 3em;  /* i.e., 36px */
}
h1 { font-size: 15px }

Absolute length

units are fixed in relation to each other. They are mainly useful when the output environment is known. The absolute units consist of the physical units (in, cm, mm, pt, pc) and the px unit:

• in: inches — 1in is equal to 2.54cm.
• cm: centimeters
• mm: millimeters
• pt: points — the points used by CSS are equal to 1/72nd of 1in.
• pc: picas — 1pc is equal to 12pt.
• px: pixel units — 1px is equal to 0.75pt.

For a CSS device, these dimensions are either anchored (i) by relating the physical units to their physical measurements, or (ii) by relating the pixel unit to the reference pixel. For print media and similar high-resolution devices, the anchor unit should be one of the standard physical units (inches, centimeters, etc). For lower-resolution devices, and devices with unusual viewing distances, it is recommended instead that the anchor unit be the pixel unit. For such devices it is recommended that the pixel unit refer to the whole number of device pixels that best approximates the reference pixel.

Note that if the anchor unit is the pixel unit, the physical units might not match their physical measurements. Alternatively if the anchor unit is a physical unit, the pixel unit might not map to a whole number of device pixels.

Note that this definition of the pixel unit and the physical units differs from previous versions of CSS. In particular, in previous versions of CSS the pixel unit and the physical units were not related by a fixed ratio: the physical units were always tied to their physical measurements while the pixel unit would vary to most closely match the reference pixel. (This change was made because too much existing content relies on the assumption of 96dpi, and breaking that assumption breaks the content.)

The reference pixel is the visual angle of one pixel on a device with a pixel density of 96dpi and a distance from the reader of an arm’s length. For a nominal arm’s length of 28 inches, the visual angle is therefore about 0.0213 degrees. For reading at arm’s length, 1px thus corresponds to about 0.26 mm (1/96 inch).

The image below illustrates the effect of viewing distance on the size of a reference pixel: a reading distance of 71 cm (28 inches) results in a reference pixel of 0.26 mm, while a reading distance of 3.5 m (12 feet) results in a reference pixel of 1.3 mm.

This second image illustrates the effect of a device’s resolution on the pixel unit: an area of 1px by 1px is covered by a single dot in a low-resolution device (e.g. a typical computer display), while the same area is covered by 16 dots in a higher resolution device (such as a printer).

h1 { margin: 0.5in }      /* inches  */
h2 { line-height: 3cm }   /* centimeters */
h3 { word-spacing: 4mm }  /* millimeters */
h4 { font-size: 12pt }    /* points */
h4 { font-size: 1pc }     /* picas */
p  { font-size: 12px }    /* px */

4.3.3. Percentages

The format of a percentage value (denoted by <percentage> in this specification) is a <number> immediately followed by %.

Percentage values are always relative to another value, for example a length. Each property that allows percentages also defines the value to which the percentage refers. The value may be that of another property for the same element, a property for an ancestor element, or a value of the formatting context (e.g., the width of a containing block). When a percentage value is set for a property of the root element and the percentage is defined as referring to the inherited value of some property, the resultant value is the percentage times the initial value of that property.

p { font-size: 10px }
p { line-height: 120% }  /* 120% of 'font-size' */

4.3.4. URLs and URIs

URI values (Uniform Resource Identifiers, see [RFC3986], which includes URLs, URNs, etc) in this specification are denoted by <uri>. The functional notation used to designate URIs in property values is "url()", as in:

body { background: url("http://www.example.com/pinkish.png") }

The format of a URI value is url( followed by optional white space followed by an optional single quote (') or double quote (") character followed by the URI itself, followed by an optional single quote (') or double quote (") character followed by optional white space followed by ). The two quote characters must be the same.

li { list-style: url(http://www.example.com/redball.png) disc }

Some characters appearing in an unquoted URI, such as parentheses, white space characters, single quotes (') and double quotes ("), must be escaped with a backslash so that the resulting URI value is a URI token: \(, \).

Depending on the type of URI, it might also be possible to write the above characters as URI-escapes (where "(" = %28, ")" = %29, etc.) as described in [RFC3986].

In order to create modular style sheets that are not dependent on the absolute location of a resource, authors may use relative URIs. Relative URIs (as defined in [RFC3986]) are resolved to full URIs using a base URI. RFC 3986, section 5, defines the normative algorithm for this process. For CSS style sheets, the base URI is that of the style sheet, not that of the source document.

body { background: url("yellow") }

http://www.example.org/style/basic.css

http://www.example.org/style/yellow

User agents may vary in how they handle invalid URIs or URIs that designate unavailable or inapplicable resources.

4.3.5. Counters

Counters are denoted by case-sensitive identifiers (see the counter-increment and counter-reset properties). To refer to the value of a counter, the notation counter(<identifier>) or 'counter(<identifier>, <list-style-type'>), with optional white space separating the tokens, is used. The default style is decimal''.

To refer to a sequence of nested counters of the same name, the notation is counters(<identifier>, <string>) or 'counters(<identifier>, <string>, <list-style-type'>)'' with optional white space separating the tokens.

See "Nested counters and scope" in the chapter on generated content for how user agents must determine the value or values of the counter. See the definition of counter values of the content property for how it must convert these values to a string.

In CSS 2, the values of counters can only be referred to from the content property. Note that none is a possible <list-style-type>: 'counter(x, none)' yields an empty string.

p {counter-increment: par-num}
h1 {counter-reset: par-num}
p:before {content: counter(par-num, upper-roman) ". "}

4.3.6. Colors

A <color> is either a keyword or a numerical RGB specification.

The list of color keywords is: aqua, black, blue, fuchsia, gray, green, lime, maroon, navy, olive, orange, purple, red, silver, teal, white, and yellow. These 17 colors have the following values:

In addition to these color keywords, users may specify keywords that correspond to the colors used by certain objects in the user’s environment. Please consult the section on system colors for more information.

body {color: black; background: white }
h1 { color: maroon }
h2 { color: olive }

The RGB color model is used in numerical color specifications. These examples all specify the same color:

em { color: #f00 }              /* #rgb */
em { color: #ff0000 }           /* #rrggbb */
em { color: rgb(255,0,0) }
em { color: rgb(100%, 0%, 0%) }

The format of an RGB value in hexadecimal notation is a # immediately followed by either three or six hexadecimal characters. The three-digit RGB notation (#rgb) is converted into six-digit form (#rrggbb) by replicating digits, not by adding zeros. For example, #fb0 expands to #ffbb00. This ensures that white (#ffffff) can be specified with the short notation (#fff) and removes any dependencies on the color depth of the display.

The format of an RGB value in the functional notation is rgb( followed by a comma-separated list of three numerical values (either three integer values or three percentage values) followed by ). The integer value 255 corresponds to 100%, and to F or FF in the hexadecimal notation: rgb(255,255,255) = rgb(100%,100%,100%) = #FFF. White space characters are allowed around the numerical values.

All RGB colors are specified in the sRGB color space (see [SRGB]). User agents may vary in the fidelity with which they represent these colors, but using sRGB provides an unambiguous and objectively measurable definition of what the color should be, which can be related to international standards (see [COLORIMETRY]).

Conforming user agents may limit their color-displaying efforts to performing a gamma-correction on them. sRGB specifies a display gamma of 2.2 under specified viewing conditions. User agents should adjust the colors given in CSS such that, in combination with an output device’s "natural" display gamma, an effective display gamma of 2.2 is produced. Note that only colors specified in CSS are affected; e.g., images are expected to carry their own color information.

Values outside the device gamut should be clipped or mapped into the gamut when the gamut is known: the red, green, and blue values must be changed to fall within the range supported by the device. User agents may perform higher quality mapping of colors from one gamut to another. For a typical CRT monitor, whose device gamut is the same as sRGB, the four rules below are equivalent:

em { color: rgb(255,0,0) }       /* integer range 0 - 255 */
em { color: rgb(300,0,0) }       /* clipped to rgb(255,0,0) */
em { color: rgb(255,-10,0) }     /* clipped to rgb(255,0,0) */
em { color: rgb(110%, 0%, 0%) }  /* clipped to rgb(100%,0%,0%) */

Other devices, such as printers, have different gamuts than sRGB; some colors outside the 0..255 sRGB range will be representable (inside the device gamut), while other colors inside the 0..255 sRGB range will be outside the device gamut and will thus be mapped.

4.3.7. Strings

Strings can either be written with double quotes or with single quotes. Double quotes cannot occur inside double quotes, unless escaped (e.g., as \" or as \22). Analogously for single quotes (e.g., "'" or "\27").

"this is a 'string'"
"this is a \"string\""
'this is a "string"'
'this is a \'string\''

A string cannot directly contain a newline. To include a newline in a string, use an escape representing the line feed character in ISO-10646 (U+000A), such as "\A" or "\00000a". This character represents the generic notion of "newline" in CSS. See the content property for an example.

It is possible to break strings over several lines, for aesthetic or other reasons, but in such a case the newline itself has to be escaped with a backslash (\). For instance, the following two selectors are exactly the same:

a[title="a not s\
o very long title"] {/*...*/}
a[title="a not so very long title"] {/*...*/}

4.3.8. Unsupported Values

If a UA does not support a particular value, it should ignore that value when parsing style sheets, as if that value was an illegal value. For example:

h3 {
  display: inline;
  display: run-in;
}

A UA that supports the run-in value for the display property will accept the first display declaration and then "write over" that value with the second display declaration. A UA that does not support the run-in value will process the first display declaration and ignore the second display declaration.

4.4. CSS style sheet representation

A CSS style sheet is a sequence of characters from the Universal Character Set (see [ISO10646]). For transmission and storage, these characters must be encoded by a character encoding that supports the set of characters available in US-ASCII (e.g., UTF-8, ISO 8859-x, SHIFT JIS, etc.). For a good introduction to character sets and character encodings, please consult the HTML 4 specification ([HTML401], chapter 5). See also the XML 1.0 specification ([XML10], sections 2.2 and 4.3.3, and Appendix F).

When a style sheet is embedded in another document, such as in the STYLE element or "style" attribute of HTML, the style sheet shares the character encoding of the whole document.

When a style sheet resides in a separate file, user agents must observe the following priorities when determining a style sheet’s character encoding (from highest priority to lowest):

1. An HTTP "charset" parameter in a "Content-Type" field (or similar parameters in other protocols)
2. BOM and/or @charset (see below)
3. <link charset=""> or other metadata from the linking mechanism (if any)
4. charset of referring style sheet or document (if any)
5. Assume UTF-8

Authors using an @charset rule must place the rule at the very beginning of the style sheet, preceded by no characters. (If a byte order mark is appropriate for the encoding used, it may precede the @charset rule.)

After "@charset", authors specify the name of a character encoding (in quotes). For example:

@charset "ISO-8859-1";

@charset must be written literally, i.e., the 10 characters '@charset "' (lowercase, no backslash escapes), followed by the encoding name, followed by ";.

The name must be a charset name as described in the IANA registry. See [CHARSETS] for a complete list of charsets. Authors should use the charset names marked as "preferred MIME name" in the IANA registry.

User agents must support at least the UTF-8 encoding.

User agents must ignore any @charset rule not at the beginning of the style sheet. When user agents detect the character encoding using the BOM and/or the @charset rule, they should follow the following rules:

• Except as specified in these rules, all @charset rules are ignored.
• The encoding is detected based on the stream of bytes that begins the style sheet. The following table gives a set of possibilities for initial byte sequences (written in hexadecimal). The first row that matches the beginning of the style sheet gives the result of encoding detection based on the BOM and/or @charset rule. If no rows match, the encoding cannot be detected based on the BOM and/or @charset rule. The notation (...)* refers to repetition for which the best match is the one that repeats as few times as possible. The bytes marked "XX" are those used to determine the name of the encoding, by treating them, in the order given, as a sequence of ASCII characters. Bytes marked "YY" are similar, but need to be transcoded into ASCII as noted. User agents may ignore entries in the table if they do not support any encodings relevant to the entry.

  Relationship between initial bytes of sheet and chosen encoding

  Initial Bytes	Result
  EF BB BF 40 63 68 61 72 73 65 74 20 22 (XX)* 22 3B	as specified
  EF BB BF	UTF-8
  40 63 68 61 72 73 65 74 20 22 (XX)* 22 3B	as specified
  FE FF 00 40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 (00 XX)* 00 22 00 3B	as specified (with BE endianness if not specified)
  00 40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 (00 XX)* 00 22 00 3B	as specified (with BE endianness if not specified)
  FF FE 40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 00 (XX 00)* 22 00 3B 00	as specified (with LE endianness if not specified)
  40 00 63 00 68 00 61 00 72 00 73 00 65 00 74 00 20 00 22 00 (XX 00)* 22 00 3B 00	as specified (with LE endianness if not specified)
  00 00 FE FF 00 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 (00 00 00 XX)* 00 00 00 22 00 00 00 3B	as specified (with BE endianness if not specified)
  00 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 (00 00 00 XX)* 00 00 00 22 00 00 00 3B	as specified (with BE endianness if not specified)
  00 00 FF FE 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 (00 00 XX 00)* 00 00 22 00 00 00 3B 00	as specified (with 2143 endianness if not specified)
  00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 (00 00 XX 00)* 00 00 22 00 00 00 3B 00	as specified (with 2143 endianness if not specified)
  FE FF 00 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 00 (00 XX 00 00)* 00 22 00 00 00 3B 00 00	as specified (with 3412 endianness if not specified)
  00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 00 (00 XX 00 00)* 00 22 00 00 00 3B 00 00	as specified (with 3412 endianness if not specified)
  FF FE 00 00 40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 00 00 (XX 00 00 00)* 22 00 00 00 3B 00 00 00	as specified (with LE endianness if not specified)
  40 00 00 00 63 00 00 00 68 00 00 00 61 00 00 00 72 00 00 00 73 00 00 00 65 00 00 00 74 00 00 00 20 00 00 00 22 00 00 00 (XX 00 00 00)* 22 00 00 00 3B 00 00 00	as specified (with LE endianness if not specified)
  00 00 FE FF	UTF-32-BE
  FF FE 00 00	UTF-32-LE
  00 00 FF FE	UTF-32-2143
  FE FF 00 00	UTF-32-3412
  FE FF	UTF-16-BE
  FF FE	UTF-16-LE
  7C 83 88 81 99 A2 85 A3 40 7F (YY)* 7F 5E	as specified, transcoded from EBCDIC to ASCII
  AE 83 88 81 99 A2 85 A3 40 FC (YY)* FC 5E	as specified, transcoded from IBM1026 to ASCII
  00 63 68 61 72 73 65 74 20 22 (YY)* 22 3B	as specified, transcoded from GSM 03.38 to ASCII
  analogous patterns	User agents may support additional, analogous, patterns if they support encodings that are not handled by the patterns here

• If the encoding is detected based on one of the entries in the table above marked "as specified", the user agent ignores the style sheet if it does not parse an appropriate @charset rule at the beginning of the stream of characters resulting from decoding in the chosen @charset. This ensures that:
  • @charset rules should only function if they are in the encoding of the style sheet,
  • byte order marks are ignored only in encodings that support a byte order mark, and
  • encoding names cannot contain newlines.

User agents must ignore style sheets in unknown encodings.

4.4.1. Referring to characters not represented in a character encoding

A style sheet may have to refer to characters that cannot be represented in the current character encoding. These characters must be written as escaped references to ISO 10646 characters. These escapes serve the same purpose as numeric character references in HTML or XML documents (see [HTML401], chapters 5 and 25).

The character escape mechanism should be used when only a few characters must be represented this way. If most of a style sheet requires escaping, authors should encode it with a more appropriate encoding (e.g., if the style sheet contains a lot of Greek characters, authors might use "ISO-8859-7" or "UTF-8").

Intermediate processors using a different character encoding may translate these escaped sequences into byte sequences of that encoding. Intermediate processors must not, on the other hand, alter escape sequences that cancel the special meaning of an ASCII character.

Conforming user agents must correctly map to ISO-10646 all characters in any character encodings that they recognize (or they must behave as if they did).

For example, a style sheet transmitted as ISO-8859-1 (Latin-1) cannot contain Greek letters directly: "κουρος" (Greek: "kouros") has to be written as "\3BA\3BF\3C5\3C1\3BF\3C2".

<SPAN style="font-family: L\FC beck">...</SPAN>

<SPAN style="font-family: L&#252;beck">...</SPAN>

5. Selectors

5.1. Pattern matching

In CSS, pattern matching rules determine which style rules apply to elements in the document tree. These patterns, called selectors, may range from simple element names to rich contextual patterns. If all conditions in the pattern are true for a certain element, the selector matches the element.

The case-sensitivity of document language element names in selectors depends on the document language. For example, in HTML, element names are case-insensitive, but in XML they are case-sensitive.

The following table summarizes CSS 2 selector syntax:

5.2. Selector syntax

A simple selector is either a type selector or universal selector followed immediately by zero or more attribute selectors, ID selectors, or pseudo-classes, in any order. The simple selector matches if all of its components match.

Note: the terminology used here in CSS 2 is different from what is used in CSS3. For example, a "simple selector" refers to a smaller part of a selector in CSS3 than in CSS 2. See the CSS3 Selectors module [SELECTORS-3].

A selector is a chain of one or more simple selectors separated by combinators. Combinators are: white space, ">", and "+". White space may appear between a combinator and the simple selectors around it.

The elements of the document tree that match a selector are called subjects of the selector. A selector consisting of a single simple selector matches any element satisfying its requirements. Prepending a simple selector and combinator to a chain imposes additional matching constraints, so the subjects of a selector are always a subset of the elements matching the last simple selector.

One pseudo-element may be appended to the last simple selector in a chain, in which case the style information applies to a subpart of each subject.

5.2.1. Grouping