Selectors

Abstract

Selectors are patterns that match against elements in a tree. Selectors have been optimized for use with HTML and XML, and are designed to be usable in performance-critical code.

CSS (Cascading Style Sheets) is a language for describing the rendering of HTML and XML documents on screen, on paper, in speech, etc. CSS uses Selectors for binding style properties to elements in the document. This document describes extensions to the selectors defined in CSS level 2. These extended selectors will be used by CSS level 3.

Selectors define the following function:

expression ∗ element → boolean

That is, given an element and a selector, this specification defines whether that element matches the selector.

These expressions can also be used, for instance, to select a set of elements, or a single element from a set of elements, by evaluating the expression across all the elements in a subtree. STTS (Simple Tree Transformation Sheets), a language for transforming XML trees, uses this mechanism. [STTS]

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/.

This document describes the selectors that already exist in CSS1 and CSS2, and also proposes new selectors for CSS3 and other languages that may need them.

The CSS Working Group doesn't expect that all implementations of CSS3 will have to implement all selectors. Instead, there will probably be a small number of variants of CSS3, called profiles. For example, it may be that only a profile for interactive user agents will include all of the selectors.

This specification is a last call working draft for the the CSS Working Group (Style Activity). This document is a revision of the Candidate Recommendation dated 2001 November 13, and has incorporated implementation feedback received in the past few years. It is expected that this last call will proceed straight to Proposed Recommendation stage since it is believed that interoperability will be demonstrable.

All persons are encouraged to review and implement this specification and return comments to the (archived) public mailing list www-style (see instructions). W3C Members can also send comments directly to the CSS Working Group. The deadline for comments is 14 January 2006.

This is still a draft document and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to cite a W3C Working Draft as other than "work in progress".

This document may be available in translation. The English version of this specification is the only normative version.

1. Introduction

1.1. Dependencies

Some features of this specification are specific to CSS, or have particular limitations or rules specific to CSS. In this specification, these have been described in terms of CSS2.1. [CSS21]

1.2. Terminology

All of the text of this specification is normative except examples, notes, and sections explicitly marked as non-normative.

1.3. Changes from CSS2

This section is non-normative.

The main differences between the selectors in CSS2 and those in Selectors are:

• the list of basic definitions (selector, group of selectors, simple selector, etc.) has been changed; in particular, what was referred to in CSS2 as a simple selector is now called a sequence of simple selectors, and the term "simple selector" is now used for the components of this sequence
• an optional namespace component is now allowed in type element selectors, the universal selector and attribute selectors
• a new combinator has been introduced
• new simple selectors including substring matching attribute selectors, and new pseudo-classes
• new pseudo-elements, and introduction of the "::" convention for pseudo-elements
• the grammar has been rewritten
• profiles to be added to specifications integrating Selectors and defining the set of selectors which is actually supported by each specification
• Selectors are now a CSS3 Module and an independent specification; other specifications can now refer to this document independently of CSS
• the specification now has its own test suite

2. Selectors

This section is non-normative, as it merely summarizes the following sections.

A Selector represents a structure. This structure can be used as a condition (e.g. in a CSS rule) that determines which elements a selector matches in the document tree, or as a flat description of the HTML or XML fragment corresponding to that structure.

Selectors may range from simple element names to rich contextual representations.

The following table summarizes the Selector syntax:

The meaning of each selector is derived from the table above by prepending "matches" to the contents of each cell in the "Meaning" column.

3. Case sensitivity

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

4. Selector syntax

A selector is a chain of one or more sequences of simple selectors separated by combinators.

A sequence of simple selectors is a chain of simple selectors that are not separated by a combinator. It always begins with a type selector or a universal selector. No other type selector or universal selector is allowed in the sequence.

A simple selector is either a type selector, universal selector, attribute selector, class selector, ID selector, content selector, or pseudo-class. One pseudo-element may be appended to the last sequence of simple selectors.

Combinators are: white space, "greater-than sign" (U+003E, >), "plus sign" (U+002B, +) and "tilde" (U+007E, ~). White space may appear between a combinator and the simple selectors around it. 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 elements of a document tree that are represented by a selector are the subjects of the selector. A selector consisting of a single sequence of simple selectors represents any element satisfying its requirements. Prepending another sequence of simple selectors and a combinator to a sequence imposes additional matching constraints, so the subjects of a selector are always a subset of the elements represented by the last sequence of simple selectors.

An empty selector, containing no sequence of simple selectors and no pseudo-element, is an invalid selector.

5. Groups of selectors

When several selectors share the same declarations, they may be grouped into a comma-separated list. (A comma is U+002C.)

h1 { font-family: sans-serif }
h2 { font-family: sans-serif }
h3 { font-family: sans-serif }

h1, h2, h3 { font-family: sans-serif }

Warning: the equivalence is true in this example because all the selectors are valid selectors. If just one of these selectors were invalid, the entire group of selectors would be invalid. This would invalidate the rule for all three heading elements, whereas in the former case only one of the three individual heading rules would be invalidated.

6. Simple selectors

6.1. Type selector

A type selector is the name of a document language element type. A type selector represents an instance of the element type in the document tree.

h1

6.1.1. Type selectors and namespaces

Type selectors allow an optional namespace ([XMLNAMES]) component. A namespace prefix that has been previously declared may be prepended to the element name separated by the namespace separator "vertical bar" (U+007C, |).

The namespace component may be left empty to indicate that the selector is only to represent elements with no declared namespace.

An asterisk may be used for the namespace prefix, indicating that the selector represents elements in any namespace (including elements with no namespace).

Element type selectors that have no namespace component (no namespace separator), represent elements without regard to the element's namespace (equivalent to "*|") unless a default namespace has been declared. If a default namespace has been declared, the selector will represent only elements in the default namespace.

A type selector containing a namespace prefix that has not been previously declared is an invalid selector. The mechanism for declaring a namespace prefix is left up to the language implementing Selectors. In CSS, such a mechanism is defined in the General Syntax module.

In a namespace-aware client, element type selectors will only match against the local part of the element's qualified name. See below for notes about matching behaviors in down-level clients.

In summary:

ns|E

elements with name E in namespace ns

*|E

elements with name E in any namespace, including those without any declared namespace

|E

elements with name E without any declared namespace

E

if no default namespace has been specified, this is equivalent to *|E. Otherwise it is equivalent to ns|E where ns is the default namespace.

@namespace foo url(http://www.example.com);
 foo|h1 { color: blue }
 foo|* { color: yellow }
 |h1 { color: red }
 *|h1 { color: green }
 h1 { color: green }

6.2. Universal selector

The universal selector, written "asterisk" (*), represents the qualified name of any element type. It represents any single element in the document tree in any namespace (including those without any declared namespace) if no default namespace has been specified. If a default namespace has been specified, see Universal selector and Namespaces below.

If the universal selector is not the only component of a sequence of simple selectors, the * may be omitted.

Note: it is recommended that the *, representing the universal selector, not be omitted.

6.2.1. Universal selector and namespaces

The universal selector allows an optional namespace component. It is used as follows:

ns|*

all elements in namespace ns

*|*

all elements

|*

all elements without any declared namespace

*

if no default namespace has been specified, this is equivalent to *|*. Otherwise it is equivalent to ns|* where ns is the default namespace.

A universal selector containing a namespace prefix that has not been previously declared is an invalid selector. The mechanism for declaring a namespace prefix is left up to the language implementing Selectors. In CSS, such a mechanism is defined in the General Syntax module.

6.3. Attribute selectors

Selectors allow the representation of an element's attributes. When a selector is used as an expression to match against an element, attribute selectors must be considered to match an element if that element has an attribute that matches the attribute represented by the attribute selector.

6.3.1. Attribute presence and values selectors

CSS2 introduced four attribute selectors:

[att]

Represents an element with the att attribute, whatever the value of the attribute.

[att=val]

Represents an element with the att attribute whose value is exactly "val".

[att~=val]

Represents an element with the att attribute whose value is a whitespace-separated list of words, one of which is exactly "val". If "val" contains whitespace, it will never represent anything (since the words are separated by spaces).

[att|=val]

Represents an element with the att attribute, its value either being exactly "val" or beginning with "val" immediately followed by "-" (U+002D). This is primarily intended to allow language subcode matches (e.g., the hreflang attribute on the link element in HTML) as described in RFC 3066 ([RFC3066]). For lang (or xml:lang) language subcode matching, please see the :lang pseudo-class.

Attribute values must be identifiers or strings. The case-sensitivity of attribute names and values in selectors depends on the document language.

h1[title]

span[class="example"]

span[hello="Cleveland"][goodbye="Columbus"]

a[rel~="copyright"]
a[href="http://www.w3.org/"]

link[hreflang=fr]

link[hreflang|="en"]

DIALOGUE[character=romeo]
DIALOGUE[character=juliet]

6.3.2. Substring matching attribute selectors

Three additional attribute selectors are provided for matching substrings in the value of an attribute:

[att^=val]

Represents an element with the att attribute whose value begins with the prefix "val".

[att$=val]

Represents an element with the att attribute whose value ends with the suffix "val".

[att*=val]

Represents an element with the att attribute whose value contains at least one instance of the substring "val".

Attribute values must be identifiers or strings. The case-sensitivity of attribute names in selectors depends on the document language.

object[type^="image/"]

a[href$=".html"]

p[title*="hello"]

6.3.3. Attribute selectors and namespaces

Attribute selectors allow an optional namespace component to the attribute name. A namespace prefix that has been previously declared may be prepended to the attribute name separated by the namespace separator "vertical bar" (|). In keeping with the Namespaces in the XML recommendation, default namespaces do not apply to attributes, therefore attribute selectors without a namespace component apply only to attributes that have no declared namespace (equivalent to "|attr"). An asterisk may be used for the namespace prefix indicating that the selector is to match all attribute names without regard to the attribute's namespace.

An attribute selector with an attribute name containing a namespace prefix that has not been previously declared is an invalid selector. The mechanism for declaring a namespace prefix is left up to the language implementing Selectors. In CSS, such a mechanism is defined in the General Syntax module.

@namespace foo "http://www.example.com";
[foo|att=val] { color: blue }
[*|att] { color: yellow }
[|att] { color: green }
[att] { color: green }

6.3.4. Default attribute values in DTDs

Attribute selectors represent explicitly set attribute values in the document tree. Default attribute values may be defined in a DTD or elsewhere, but cannot always be selected by attribute selectors. Selectors should be designed so that they work even if the default values are not included in the document tree.

More precisely, a UA is not required to read an "external subset" of the DTD but is required to look for default attribute values in the document's "internal subset." (See [XML10] for definitions of these subsets.)

A UA that recognizes an XML namespace [XMLNAMES] is not required to use its knowledge of that namespace to treat default attribute values as if they were present in the document. (For example, an XHTML UA is not required to use its built-in knowledge of the XHTML DTD.)

Note: Typically, implementations choose to ignore external subsets.

<!ATTLIST EXAMPLE notation (decimal,octal) "decimal">

EXAMPLE[notation=decimal] { /*... default property settings ...*/ }
EXAMPLE[notation=octal]   { /*... other settings...*/ }

EXAMPLE                   { /*... default property settings ...*/ }
EXAMPLE[notation=octal]   { /*... other settings...*/ }

6.4. Class selectors

Working with HTML, authors may use the period (U+002E, .) notation as an alternative to the ~= notation when representing the class attribute. Thus, for HTML, div.value and div[class~=value] have the same meaning. The attribute value must immediately follow the "period" (.).

UAs may apply selectors using the period (.) notation in XML documents if the UA has namespace-specific knowledge that allows it to determine which attribute is the "class" attribute for the respective namespace. One such example of namespace-specific knowledge is the prose in the specification for a particular namespace (e.g. SVG 1.0 [SVG] describes the SVG "class" attribute and how a UA should interpret it, and similarly MathML 1.01 [MATH] describes the MathML "class" attribute.)

*.pastoral { color: green }  /* all elements with class~=pastoral */

.pastoral { color: green }  /* all elements with class~=pastoral */

H1.pastoral { color: green }  /* H1 elements with class~=pastoral */

<H1>Not green</H1>
<H1 class="pastoral">Very green</H1>

To represent a subset of "class" values, each value must be preceded by a ".", in any order.

p.pastoral.marine { color: green }

Note: Because CSS gives considerable power to the "class" attribute, authors could conceivably design their own "document language" based on elements with almost no associated presentation (such as DIV and SPAN in HTML) and assigning style information through the "class" attribute. Authors should avoid this practice since the structural elements of a document language often have recognized and accepted meanings and author-defined classes may not.

Note: If an element has multiple class attributes, their values must be concatenated with spaces between the values before searching for the class. As of this time the working group is not aware of any manner in which this situation can be reached, however, so this behavior is explicitly non-normative in this specification.

6.5. ID selectors

Document languages may contain attributes that are declared to be of type ID. What makes attributes of type ID special is that no two such attributes can have the same value in a document, regardless of the type of the elements that carry them; whatever the document language, an ID typed attribute can be used to uniquely identify its element. In HTML all ID attributes are named "id"; XML applications may name ID attributes differently, but the same restriction applies.

An ID-typed attribute of a document language allows authors to assign an identifier to one element instance in the document tree. W3C ID selectors represent an element instance based on its identifier. An ID selector contains a "number sign" (U+0023, #) immediately followed by the ID value, which must be an identifier.

Selectors does not specify how a UA knows the ID-typed attribute of an element. The UA may, e.g., read a document's DTD, have the information hard-coded or ask the user.

h1#chapter1

#chapter1

*#z98y

Note. In XML 1.0 [XML10], the information about which attribute contains an element's IDs is contained in a DTD or a schema. When parsing XML, UAs do not always read the DTD, and thus may not know what the ID of an element is (though a UA may have namespace-specific knowledge that allows it to determine which attribute is the ID attribute for that namespace). If a style sheet designer knows or suspects that a UA may not know what the ID of an element is, he should use normal attribute selectors instead: [name=p371] instead of #p371. Elements in XML 1.0 documents without a DTD do not have IDs at all.

If an element has multiple ID attributes, all of them must be treated as IDs for that element for the purposes of the ID selector. Such a situation could be reached using mixtures of xml:id, DOM3 Core, XML DTDs, and namespace-specific knowledge.

6.6. Pseudo-classes

The pseudo-class concept is introduced to permit selection based on information that lies outside of the document tree or that cannot be expressed using the other simple selectors.

A pseudo-class always consists of a "colon" (:) followed by the name of the pseudo-class and optionally by a value between parentheses.

Pseudo-classes are allowed in all sequences of simple selectors contained in a selector. Pseudo-classes are allowed anywhere in sequences of simple selectors, after the leading type selector or universal selector (possibly omitted). Pseudo-class names are case-insensitive. Some pseudo-classes are mutually exclusive, while others can be applied simultaneously to the same element. Pseudo-classes may be dynamic, in the sense that an element may acquire or lose a pseudo-class while a user interacts with the document.

6.6.1. Dynamic pseudo-classes

Dynamic pseudo-classes classify elements on characteristics other than their name, attributes, or content, in principle characteristics that cannot be deduced from the document tree.

Dynamic pseudo-classes do not appear in the document source or document tree.

The link pseudo-classes: :link and :visited

User agents commonly display unvisited links differently from previously visited ones. Selectors provides the pseudo-classes :link and :visited to distinguish them:

• The :link pseudo-class applies to links that have not yet been visited.
• The :visited pseudo-class applies once the link has been visited by the user.

After some amount of time, user agents may choose to return a visited link to the (unvisited) ':link' state.

The two states are mutually exclusive.

a.external:visited

Note: It is possible for style sheet authors to abuse the :link and :visited pseudo-classes to determine which sites a user has visited without the user's consent.

UAs may therefore treat all links as unvisited links, or implement other measures to preserve the user's privacy while rendering visited and unvisited links differently.

The user action pseudo-classes :hover, :active, and :focus

Interactive user agents sometimes change the rendering in response to user actions. Selectors provides three pseudo-classes for the selection of an element the user is acting on.

• The :hover pseudo-class applies while the user designates an element with a pointing device, but does not activate it. For example, a visual user agent could apply this pseudo-class when the cursor (mouse pointer) hovers over a box generated by the element. User agents not that do not support interactive media do not have to support this pseudo-class. Some conforming user agents that support interactive media may not be able to support this pseudo-class (e.g., a pen device that does not detect hovering).
• The :active pseudo-class applies while an element is being activated by the user. For example, between the times the user presses the mouse button and releases it.
• The :focus pseudo-class applies while an element has the focus (accepts keyboard or mouse events, or other forms of input).

There may be document language or implementation specific limits on which elements can become :active or acquire :focus.

These pseudo-classes are not mutually exclusive. An element may match several pseudo-classes at the same time.

Selectors doesn't define if the parent of an element that is ':active' or ':hover' is also in that state.

a:link    /* unvisited links */
a:visited /* visited links */
a:hover   /* user hovers */
a:active  /* active links */

a:focus
a:focus:hover

Note: An element can be both ':visited' and ':active' (or ':link' and ':active').

6.6.2. The target pseudo-class :target

Some URIs refer to a location within a resource. This kind of URI ends with a "number sign" (#) followed by an anchor identifier (called the fragment identifier).

URIs with fragment identifiers link to a certain element within the document, known as the target element. For instance, here is a URI pointing to an anchor named section_2 in an HTML document:

http://example.com/html/top.html#section_2

A target element can be represented by the :target pseudo-class. If the document's URI has no fragment identifier, then the document has no target element.

p.note:target

*:target { color : red }
*:target::before { content : url(target.png) }

6.6.3. The language pseudo-class :lang

If the document language specifies how the human language of an element is determined, it is possible to write selectors that represent an element based on its language. For example, in HTML [HTML4], the language is determined by a combination of the lang attribute, the meta element, and possibly by information from the protocol (such as HTTP headers). XML uses an attribute called xml:lang, and there may be other document language-specific methods for determining the language.

The pseudo-class :lang(C) represents an element that is in language C. Whether an element is represented by a :lang() selector is based solely on the identifier C being either equal to, or a hyphen-separated substring of, the element's language value, in the same way as if performed by the '|=' operator in attribute selectors. The identifier C does not have to be a valid language name.

C must not be empty. (If it is, the selector is invalid.)

Note: It is recommended that documents and protocols indicate language using codes from RFC 3066 [RFC3066] or its successor, and by means of "xml:lang" attributes in the case of XML-based documents [XML10]. See "FAQ: Two-letter or three-letter language codes."

html:lang(fr-be)
html:lang(de)
:lang(fr-be) > q
:lang(de) > q

6.6.4. The UI element states pseudo-classes

The :enabled and :disabled pseudo-classes

The :enabled pseudo-class allows authors to customize the look of user interface elements that are enabled — which the user can select or activate in some fashion (e.g. clicking on a button with a mouse). There is a need for such a pseudo-class because there is no way to programmatically specify the default appearance of say, an enabled input element without also specifying what it would look like when it was disabled.

Similar to :enabled, :disabled allows the author to specify precisely how a disabled or inactive user interface element should look.

Most elements will be neither enabled nor disabled. An element is enabled if the user can either activate it or transfer the focus to it. An element is disabled if it could be enabled, but the user cannot presently activate it or transfer focus to it.

The :checked pseudo-class

Radio and checkbox elements can be toggled by the user. Some menu items are "checked" when the user selects them. When such elements are toggled "on" the :checked pseudo-class applies. The :checked pseudo-class initially applies to such elements that have the HTML4 selected and checked attributes as described in Section 17.2.1 of HTML4, but of course the user can toggle "off" such elements in which case the :checked pseudo-class would no longer apply. While the :checked pseudo-class is dynamic in nature, and is altered by user action, since it can also be based on the presence of the semantic HTML4 selected and checked attributes, it applies to all media.

The :indeterminate pseudo-class

6.6.5. Structural pseudo-classes

Selectors introduces the concept of structural pseudo-classes to permit selection based on extra information that lies in the document tree but cannot be represented by other simple selectors or combinators.

Note that standalone pieces of PCDATA (text nodes in the DOM) are not counted when calculating the position of an element in the list of children of its parent. When calculating the position of an element in the list of children of its parent, the index numbering starts at 1.

:root pseudo-class

The :root pseudo-class represents an element that is the root of the document. In HTML 4, this is always the HTML element.

:nth-child() pseudo-class

The :nth-child(an+b) pseudo-class notation represents an element that has an+b-1 siblings before it in the document tree, for a given positive integer or zero value of n, and has a parent element. In other words, this matches the bth child of an element after all the children have been split into groups of a elements each. For example, this allows the selectors to address every other row in a table, and could be used to alternate the color of paragraph text in a cycle of four. The a and b values must be zero, negative integers or positive integers. The index of the first child of an element is 1.

In addition to this, :nth-child() can take 'odd' and 'even' as arguments instead. 'odd' has the same signification as 2n+1, and 'even' has the same signification as 2n.

tr:nth-child(2n+1) /* represents every odd row of an HTML table */
tr:nth-child(odd)  /* same */
tr:nth-child(2n)   /* represents every even row of an HTML table */
tr:nth-child(even) /* same */
/* Alternate paragraph colours in CSS */
p:nth-child(4n+1) { color: navy; }
p:nth-child(4n+2) { color: green; }
p:nth-child(4n+3) { color: maroon; }
p:nth-child(4n+4) { color: purple; }

When a=0, no repeating is used, so for example :nth-child(0n+5) matches only the fifth child. When a=0, the an part need not be included, so the syntax simplifies to :nth-child(b) and the last example simplifies to :nth-child(5).

foo:nth-child(0n+1)   /* represents an element foo, first child of its parent element */
foo:nth-child(1)      /* same */

When a=1, the number may be omitted from the rule.

bar:nth-child(1n+0)   /* represents all bar elements, specificity (0,1,1) */
bar:nth-child(n+0)    /* same */
bar:nth-child(n)      /* same */
bar                   /* same but lower specificity (0,0,1) */

If b=0, then every ath element is picked. In such a case, the b part may be omitted.

tr:nth-child(2n+0) /* represents every even row of an HTML table */
tr:nth-child(2n) /* same */

If both a and b are equal to zero, the pseudo-class represents no element in the document tree.

The value a can be negative, but only the positive values of an+b, for n≥0, may represent an element in the document tree.

html|tr:nth-child(-n+6)  /* represents the 6 first rows of XHTML tables */

When the value b is negative, the "+" character in the expression must be removed (it is effectively replaced by the "-" character indicating the negative value of b).

:nth-child(10n-1)  /* represents the 9th, 19th, 29th, etc, element */
:nth-child(10n+9)  /* Same */
:nth-child(10n+-1) /* Syntactically invalid, and would be ignored */

:nth-last-child() pseudo-class

The :nth-last-child(an+b) pseudo-class notation represents an element that has an+b-1 siblings after it in the document tree, for a given positive integer or zero value of n, and has a parent element. See :nth-child() pseudo-class for the syntax of its argument. It also accepts the 'even' and 'odd' values as arguments.

tr:nth-last-child(-n+2)    /* represents the two last rows of an HTML table */
foo:nth-last-child(odd)    /* represents all odd foo elements in their parent element,
                              counting from the last one */

:nth-of-type() pseudo-class

The :nth-of-type(an+b) pseudo-class notation represents an element that has an+b-1 siblings with the same element name before it in the document tree, for a given zero or positive integer value of n, and has a parent element. In other words, this matches the bth child of that type after all the children of that type have been split into groups of a elements each. See :nth-child() pseudo-class for the syntax of its argument. It also accepts the 'even' and 'odd' values.

img:nth-of-type(2n+1) { float: right; }
img:nth-of-type(2n) { float: left; }

:nth-last-of-type() pseudo-class

The :nth-last-of-type(an+b) pseudo-class notation represents an element that has an+b-1 siblings with the same element name after it in the document tree, for a given zero or positive integer value of n, and has a parent element. See :nth-child() pseudo-class for the syntax of its argument. It also accepts the 'even' and 'odd' values.

body > h2:nth-of-type(n+2):nth-last-of-type(n+2)

body > h2:not(:first-of-type):not(:last-of-type)

:first-child pseudo-class

Same as :nth-child(1). The :first-child pseudo-class represents an element that is the first child of some other element.

div > p:first-child

<p> The last P before the note.</p>
<div class="note">
   <p> The first P inside the note.</p>
</div>

<p> The last P before the note.</p>
<div class="note">
   <h2> Note </h2>
   <p> The first P inside the note.</p>
</div>

* > a:first-child /* a is first child of any element */
a:first-child /* Same (assuming a is not the root element) */

:last-child pseudo-class

Same as :nth-last-child(1). The :last-child pseudo-class represents an element that is the last child of some other element.

ol > li:last-child

:first-of-type pseudo-class

Same as :nth-of-type(1). The :first-of-type pseudo-class represents an element that is the first sibling of its type in the list of children of its parent element.

dl dt:first-of-type

<dl>
 <dt>gigogne</dt>
 <dd>
  <dl>
   <dt>fusée</dt>
   <dd>multistage rocket</dd>
   <dt>table</dt>
   <dd>nest of tables</dd>
  </dl>
 </dd>
</dl>

:last-of-type pseudo-class

Same as :nth-last-of-type(1). The :last-of-type pseudo-class represents an element that is the last sibling of its type in the list of children of its parent element.

tr > td:last-of-type

:only-child pseudo-class

Represents an element that has a parent element and whose parent element has no other element children. Same as :first-child:last-child or :nth-child(1):nth-last-child(1), but with a lower specificity.

:only-of-type pseudo-class

Represents an element that has a parent element and whose parent element has no other element children with the same element name. Same as :first-of-type:last-of-type or :nth-of-type(1):nth-last-of-type(1), but with a lower specificity.

:empty pseudo-class

The :empty pseudo-class represents an element that has no children at all. In terms of the DOM, only element nodes and text nodes (including CDATA nodes and entity references) whose data has a non-zero length must be considered as affecting emptiness; comments, PIs, and other nodes must not affect whether an element is considered empty or not.

<p></p>

<foo>bar</foo>

<foo><bar>bla</bar></foo>

<foo>this is not <bar>:empty</bar></foo>

6.6.6. Blank

This section intentionally left blank.

6.6.7. The negation pseudo-class

The negation pseudo-class, :not(X), is a functional notation taking a simple selector (excluding the negation pseudo-class itself and pseudo-elements) as an argument. It represents an element that is not represented by the argument.

button:not([DISABLED])

*:not(FOO)

html|*:not(:link):not(:visited)

Default namespace declarations do not affect the argument of the negation pseudo-class unless the argument is a universal selector or a type selector.

*|*:not(*)

*|*:not(:hover)

Note: the :not() pseudo allows useless selectors to be written. For instance :not(*|*), which represents no element at all, or foo:not(bar), which is equivalent to foo but with a higher specificity.

7. Pseudo-elements

Pseudo-elements create abstractions about the document tree beyond those specified by the document language. For instance, document languages do not offer mechanisms to access the first letter or first line of an element's content. Pseudo-elements allow designers to refer to this otherwise inaccessible information. Pseudo-elements may also provide designers a way to refer to content that does not exist in the source document (e.g., the ::before and ::after pseudo-elements give access to generated content).

A pseudo-element is made of two colons (::) followed by the name of the pseudo-element.

This :: notation is introduced by the current document in order to establish a discrimination between pseudo-classes and pseudo-elements. For compatibility with existing style sheets, user agents must also accept the previous one-colon notation for pseudo-elements introduced in CSS levels 1 and 2 (namely, :first-line, :first-letter, :before and :after). This compatibility is not allowed for the new pseudo-elements introduced in CSS level 3.

Only one pseudo-element may appear per selector, and if present it must appear after the sequence of simple selectors that represents the subjects of the selector. A future version of this specification may allow multiple pesudo-elements per selector.

7.1. The ::first-line pseudo-element

The ::first-line pseudo-element describes the contents of the first formatted line of an element.

p::first-line { text-transform: uppercase }

The selector p::first-line does not match any real HTML element. It does match a pseudo-element that conforming user agents will insert at the beginning of every paragraph.

Note that the length of the first line depends on a number of factors, including the width of the page, the font size, etc. Thus, an ordinary HTML paragraph such as:

<P>This is a somewhat long HTML 
paragraph that will be broken into several 
lines. The first line will be identified
by a fictional tag sequence. The other lines 
will be treated as ordinary lines in the 
paragraph.</P>

the lines of which happen to be broken as follows:

THIS IS A SOMEWHAT LONG HTML PARAGRAPH THAT
will be broken into several lines. The first
line will be identified by a fictional tag 
sequence. The other lines will be treated as 
ordinary lines in the paragraph.

This paragraph might be "rewritten" by user agents to include the fictional tag sequence for ::first-line. This fictional tag sequence helps to show how properties are inherited.

<P><P::first-line> This is a somewhat long HTML 
paragraph that </P::first-line> will be broken into several
lines. The first line will be identified 
by a fictional tag sequence. The other lines 
will be treated as ordinary lines in the 
paragraph.</P>

If a pseudo-element breaks up a real element, the desired effect can often be described by a fictional tag sequence that closes and then re-opens the element. Thus, if we mark up the previous paragraph with a span element:

<P><SPAN class="test"> This is a somewhat long HTML
paragraph that will be broken into several
lines.</SPAN> The first line will be identified
by a fictional tag sequence. The other lines 
will be treated as ordinary lines in the 
paragraph.</P>

the user agent could simulate start and end tags for span when inserting the fictional tag sequence for ::first-line.

<P><P::first-line><SPAN class="test"> This is a
somewhat long HTML
paragraph that will </SPAN></P::first-line><SPAN class="test"> be
broken into several
lines.</SPAN> The first line will be identified
by a fictional tag sequence. The other lines
will be treated as ordinary lines in the 
paragraph.</P>

In CSS, the ::first-line pseudo-element can only be attached to a block-level element, an inline-block, a table-caption, or a table-cell.

The "first formatted line" of an element may occur inside a block-level descendant in the same flow (i.e., a block-level descendant that is not positioned and not a float). E.g., the first line of the div in <DIV><P>This line...</P></DIV> is the first line of the p (assuming that both p and div are block-level).

The first line of a table-cell or inline-block cannot be the first formatted line of an ancestor element. Thus, in <DIV><P STYLE="display: inline-block">Hello<BR>Goodbye</P> etcetera</DIV> the first formatted line of the div is not the line "Hello".

Note that the first line of the p in this fragment: <p><br>First... doesn't contain any letters (assuming the default style for br in HTML 4). The word "First" is not on the first formatted line.

A UA should act as if the fictional start tags of the ::first-line pseudo-elements were nested just inside the innermost enclosing block-level element. (Since CSS1 and CSS2 were silent on this case, authors should not rely on this behavior.) Here is an example. The fictional tag sequence for

<DIV>
  <P>First paragraph</P>
  <P>Second paragraph</P>
</DIV>

is

<DIV>
  <P><DIV::first-line><P::first-line>First paragraph</P::first-line></DIV::first-line></P>
  <P><P::first-line>Second paragraph</P::first-line></P>
</DIV>

The ::first-line pseudo-element is similar to an inline-level element, but with certain restrictions. In CSS, the following properties apply to a ::first-line pseudo-element: font properties, color property, background properties, 'word-spacing', 'letter-spacing', 'text-decoration', 'vertical-align', 'text-transform', 'line-height'. UAs may apply other properties as well.

7.2. The ::first-letter pseudo-element

The ::first-letter pseudo-element represents the first letter of the first line of a block, if it is not preceded by any other content (such as images or inline tables) on its line. The ::first-letter pseudo-element may be used for "initial caps" and "drop caps", which are common typographical effects. This type of initial letter is similar to an inline-level element if its 'float' property is 'none'; otherwise, it is similar to a floated element.

In CSS, these are the properties that apply to ::first-letter pseudo-elements: font properties, 'text-decoration', 'text-transform', 'letter-spacing', 'word-spacing' (when appropriate), 'line-height', 'float', 'vertical-align' (only if 'float' is 'none'), margin properties, padding properties, border properties, color property, background properties. UAs may apply other properties as well. To allow UAs to render a typographically correct drop cap or initial cap, the UA may choose a line-height, width and height based on the shape of the letter, unlike for normal elements.

p { line-height: 1.1 }
p::first-letter { font-size: 3em; font-weight: normal }
span { font-weight: bold }
...
<p><span>Het hemelsche</span> gerecht heeft zich ten lange lesten<br>
Erbarremt over my en mijn benaeuwde vesten<br>
En arme burgery, en op mijn volcx gebed<br>
En dagelix geschrey de bange stad ontzet.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
 <HEAD>
  <TITLE>Drop cap initial letter</TITLE>
  <STYLE type="text/css">
   P               { font-size: 12pt; line-height: 1.2 }
   P::first-letter { font-size: 200%; font-weight: bold; float: left }
   SPAN            { text-transform: uppercase }
  </STYLE>
 </HEAD>
 <BODY>
  <P><SPAN>The first</SPAN> few words of an article
    in The Economist.</P>
 </BODY>
</HTML>

<P>
<SPAN>
<P::first-letter>
T
</P::first-letter>he first
</SPAN> 
few words of an article in the Economist.
</P>

In order to achieve traditional drop caps formatting, user agents may approximate font sizes, for example to align baselines. Also, the glyph outline may be taken into account when formatting.

Punctuation (i.e, characters defined in Unicode in the "open" (Ps), "close" (Pe), "initial" (Pi). "final" (Pf) and "other" (Po) punctuation classes), that precedes or follows the first letter should be included. [UNICODE]

The ::first-letter also applies if the first letter is in fact a digit, e.g., the "6" in "67 million dollars is a lot of money."

In CSS, the ::first-letter pseudo-element applies to block, list-item, table-cell, table-caption, and inline-block elements. A future version of this specification may allow this pesudo-element to apply to more element types.

The ::first-letter pseudo-element can be used with all such elements that contain text, or that have a descendant in the same flow that contains text. A UA should act as if the fictional start tag of the ::first-letter pseudo-element is just before the first text of the element, even if that first text is in a descendant.

<div>
<p>The first text.

<div>
<p><div::first-letter><p::first-letter>T</...></...>he first text.

The first letter of a table-cell or inline-block cannot be the first letter of an ancestor element. Thus, in <DIV><P STYLE="display: inline-block">Hello<BR>Goodbye</P> etcetera</DIV> the first letter of the div is not the letter "H". In fact, the div doesn't have a first letter.

The first letter must occur on the first formatted line. For example, in this fragment: <p><br>First... the first line doesn't contain any letters and ::first-letter doesn't match anything (assuming the default style for br in HTML 4). In particular, it does not match the "F" of "First."

In CSS, if an element is a list item ('display: list-item'), the ::first-letter applies to the first letter in the principal box after the marker. UAs may ignore ::first-letter on list items with 'list-style-position: inside'. If an element has ::before or ::after content, the ::first-letter applies to the first letter of the element including that content.

Some languages may have specific rules about how to treat certain letter combinations. In Dutch, for example, if the letter combination "ij" appears at the beginning of a word, both letters should be considered within the ::first-letter pseudo-element.

If the letters that would form the ::first-letter are not in the same element, such as "'T" in <p>'<em>T..., the UA may create a ::first-letter pseudo-element from one of the elements, both elements, or simply not create a pseudo-element.

Similarly, if the first letter(s) of the block are not at the start of the line (for example due to bidirectional reordering), then the UA need not create the pseudo-element(s).

p { color: red; font-size: 12pt }
p::first-letter { color: green; font-size: 200% }
p::first-line { color: blue }
<P>Some text that ends up on two lines</P>

<P>
<P::first-line>
<P::first-letter> 
S 
</P::first-letter>ome text that 
</P::first-line> 
ends up on two lines 
</P>

7.3. The ::selection pseudo-element

The ::selection pseudo-element applies to the portion of a document that has been highlighted by the user. This also applies, for example, to selected text within an editable text field. This pseudo-element should not be confused with the :checked pseudo-class (which used to be named :selected)

Although the ::selection pseudo-element is dynamic in nature, and is altered by user action, it is reasonable to expect that when a UA re-renders to a static medium (such as a printed page, see [CSS21]) which was originally rendered to a dynamic medium (like screen), the UA may wish to transfer the current ::selection state to that other medium, and have all the appropriate formatting and rendering take effect as well. This is not required — UAs may omit the ::selection pseudo-element for static media.

These are the CSS properties that apply to ::selection pseudo-elements: color, background, cursor (optional), outline (optional). The computed value of the 'background-image' property on ::selection may be ignored.

7.4. The ::before and ::after pseudo-elements

The ::before and ::after pseudo-elements can be used to describe generated content before or after an element's content. They are explained in CSS 2.1 [CSS21].

When the ::first-letter and ::first-line pseudo-elements are combined with ::before and ::after, they apply to the first letter or line of the element including the inserted text.

8. Combinators

8.1. Descendant combinator

At times, authors may want selectors to describe an element that is the descendant of another element in the document tree (e.g., "an EM element that is contained within an H1 element"). Descendant combinators express such a relationship. A descendant combinator is white space that separates two sequences of simple selectors. A selector of the form "A B" represents an element B that is an arbitrary descendant of some ancestor element A.

h1 em

<h1>This <span class="myclass">headline
is <em>very</em> important</span></h1>

div * p

div p *[href]

8.2. Child combinators

A child combinator describes a childhood relationship between two elements. A child combinator is made of the "greater-than sign" (>) character and separates two sequences of simple selectors.

body > p

div ol>li p

For information on selecting the first child of an element, please see the section on the :first-child pseudo-class above.

8.3. Sibling combinators

There are two different sibling combinators: the adjacent sibling combinator and the general sibling combinator. In both cases, non-element nodes (e.g. text between elements) are ignored when considering adjacency of elements.

8.3.1. Adjacent sibling combinator

The adjacent sibling combinator is made of the "plus sign" (U+002B, +) character that separates two sequences of simple selectors. The elements represented by the two sequences share the same parent in the document tree and the element represented by the first sequence immediately precedes the element represented by the second one.

math + p

h1.opener + h2

8.3.2. General sibling combinator

The general sibling combinator is made of the "tilde" (U+007E, ~) character that separates two sequences of simple selectors. The elements represented by the two sequences share the same parent in the document tree and the element represented by the first sequence precedes (not necessarily immediately) the element represented by the second one.

h1 ~ pre

<h1>Definition of the function a</h1>
<p>Function a(x) has to be applied to all figures in the table.</p>
<pre>function a(x) = 12x/13.5</pre>

9. Calculating a selector's specificity

A selector's specificity is calculated as follows:

• count the number of ID selectors in the selector (= a)
• count the number of class selectors, attributes selectors, and pseudo-classes in the selector (= b)
• count the number of element names in the selector (= c)
• ignore pseudo-elements

Selectors inside the negation pseudo-class are counted like any other, but the negation itself does not count as a pseudo-class.

Concatenating the three numbers a-b-c (in a number system with a large base) gives the specificity.

*               /* a=0 b=0 c=0 -> specificity =   0 */
LI              /* a=0 b=0 c=1 -> specificity =   1 */
UL LI           /* a=0 b=0 c=2 -> specificity =   2 */
UL OL+LI        /* a=0 b=0 c=3 -> specificity =   3 */
H1 + *[REL=up]  /* a=0 b=1 c=1 -> specificity =  11 */
UL OL LI.red    /* a=0 b=1 c=3 -> specificity =  13 */
LI.red.level    /* a=0 b=2 c=1 -> specificity =  21 */
#x34y           /* a=1 b=0 c=0 -> specificity = 100 */
#s12:not(FOO)   /* a=1 b=0 c=1 -> specificity = 101 */

Note: the specificity of the styles specified in an HTML style attribute is described in CSS 2.1. [CSS21].

10. The grammar of Selectors

10.1. Grammar

The grammar below defines the syntax of Selectors. It is globally LL(1) and can be locally LL(2) (but note that most UA's should not use it directly, since it doesn't express the parsing conventions). The format of the productions is optimized for human consumption and some shorthand notations beyond Yacc (see [YACC]) are used:

• *: 0 or more
• +: 1 or more
• ?: 0 or 1
• |: separates alternatives
• [ ]: grouping

The productions are:

selectors_group
  : selector [ COMMA S* selector ]*
  ;
selector
  : simple_selector_sequence [ combinator simple_selector_sequence ]*
  ;
combinator
  /* combinators can be surrounded by white space */
  : PLUS S* | GREATER S* | TILDE S* | S+
  ;
simple_selector_sequence
  : [ type_selector | universal ]
    [ HASH | class | attrib | pseudo | negation ]*
  | [ HASH | class | attrib | pseudo | negation ]+
  ;
type_selector
  : [ namespace_prefix ]? element_name
  ;
namespace_prefix
  : [ IDENT | '*' ]? '|'
  ;
element_name
  : IDENT
  ;
universal
  : [ namespace_prefix ]? '*'
  ;
class
  : '.' IDENT
  ;
attrib
  : '[' S* [ namespace_prefix ]? IDENT S*
        [ [ PREFIXMATCH |
            SUFFIXMATCH |
            SUBSTRINGMATCH |
            '=' |
            INCLUDES |
            DASHMATCH ] S* [ IDENT | STRING ] S*
        ]? ']'
  ;
pseudo
  /* '::' starts a pseudo-element, ':' a pseudo-class */
  /* Exceptions: :first-line, :first-letter, :before and :after. */
  /* Note that pseudo-elements are restricted to one per selector and */
  /* occur only in the last simple_selector_sequence. */
  : ':' ':'? [ IDENT | functional_pseudo ]
  ;
functional_pseudo
  : FUNCTION S* expression ')'
  ;
expression
  /* In CSS3, the expressions are identifiers, strings, */
  /* or of the form "an+b" */
  : [ [ PLUS | '-' | DIMENSION | NUMBER | STRING | IDENT ] S* ]+
  ;
negation
  : NOT S* negation_arg S* ')'
  ;
negation_arg
  : type_selector | universal | HASH | class | attrib | pseudo
  ;

10.2. Lexical scanner

The following is the tokenizer, written in Flex (see [FLEX]) notation. The tokenizer is case-insensitive.

The two occurrences of "\377" represent the highest character number that current versions of Flex can deal with (decimal 255). They should be read as "\4177777" (decimal 1114111), which is the highest possible code point in Unicode/ISO-10646. [UNICODE]

%option case-insensitive
ident     [-]?{nmstart}{nmchar}*
name      {nmchar}+
nmstart   [_a-z]|{nonascii}|{escape}
nonascii  [^\0-\177]
unicode   \\[0-9a-f]{1,6}(\r\n|[ \n\r\t\f])?
escape    {unicode}|\\[^\n\r\f0-9a-f]
nmchar    [_a-z0-9-]|{nonascii}|{escape}
num       [0-9]+|[0-9]*\.[0-9]+
string    {string1}|{string2}
string1   \"([^\n\r\f\\"]|\\{nl}|{nonascii}|{escape})*\"
string2   \'([^\n\r\f\\']|\\{nl}|{nonascii}|{escape})*\'
invalid   {invalid1}|{invalid2}
invalid1  \"([^\n\r\f\\"]|\\{nl}|{nonascii}|{escape})*
invalid2  \'([^\n\r\f\\']|\\{nl}|{nonascii}|{escape})*
nl        \n|\r\n|\r|\f
w         [ \t\r\n\f]*
%%
[ \t\r\n\f]+     return S;
"~="             return INCLUDES;
"|="             return DASHMATCH;
"^="             return PREFIXMATCH;
"$="             return SUFFIXMATCH;
"*="             return SUBSTRINGMATCH;
{ident}          return IDENT;
{string}         return STRING;
{ident}"("       return FUNCTION;
{num}            return NUMBER;
"#"{name}        return HASH;
{w}"+"           return PLUS;
{w}">"           return GREATER;
{w}","           return COMMA;
{w}"~"           return TILDE;
":not("          return NOT;
@{ident}         return ATKEYWORD;
{invalid}        return INVALID;
{num}%           return PERCENTAGE;
{num}{ident}     return DIMENSION;
"<!--"           return CDO;
"-->"            return CDC;
"url("{w}{string}{w}")"                           return URI;
"url("{w}([!#$%&*-~]|{nonascii}|{escape})*{w}")"  return URI;
U\+[0-9a-f?]{1,6}(-[0-9a-f]{1,6})?                return UNICODE_RANGE;
\/\*[^*]*\*+([^/*][^*]*\*+)*\/                    /* ignore comments */
.                return *yytext;

11. Namespaces and down-level clients

An important issue is the interaction of CSS selectors with XML documents in web clients that were produced prior to this document. Unfortunately, due to the fact that namespaces must be matched based on the URI which identifies the namespace, not the namespace prefix, some mechanism is required to identify namespaces in CSS by their URI as well. Without such a mechanism, it is impossible to construct a CSS style sheet which will properly match selectors in all cases against a random set of XML documents. However, given complete knowledge of the XML document to which a style sheet is to be applied, and a limited use of namespaces within the XML document, it is possible to construct a style sheet in which selectors would match elements and attributes correctly.

It should be noted that a down-level CSS client will (if it properly conforms to CSS forward compatible parsing rules) ignore all @namespace at-rules, as well as all style rules that make use of namespace qualified element type or attribute selectors. The syntax of delimiting namespace prefixes in CSS was deliberately chosen so that down-level CSS clients would ignore the style rules rather than possibly match them incorrectly.

The use of default namespaces in CSS makes it possible to write element type selectors that will function in both namespace aware CSS clients as well as down-level clients. It should be noted that down-level clients may incorrectly match selectors against XML elements in other namespaces.

The following are scenarios and examples in which it is possible to construct style sheets which would function properly in web clients that do not implement this proposal.

1. The XML document does not use namespaces.

   • In this case, it is obviously not necessary to declare or use namespaces in the style sheet. Standard CSS element type and attribute selectors will function adequately in a down-level client.
   • In a CSS namespace aware client, the default behavior of element selectors matching without regard to namespace will function properly against all elements, since no namespaces are present. However, the use of specific element type selectors that match only elements that have no namespace ("|name") will guarantee that selectors will match only XML elements that do not have a declared namespace.

2. The XML document defines a single, default namespace used throughout the document. No namespace prefixes are used in element names.

   • In this case, a down-level client will function as if namespaces were not used in the XML document at all. Standard CSS element type and attribute selectors will match against all elements.

3. The XML document does not use a default namespace, all namespace prefixes used are known to the style sheet author, and there is a direct mapping between namespace prefixes and namespace URIs. (A given prefix may only be mapped to one namespace URI throughout the XML document; there may be multiple prefixes mapped to the same URI).

   • In this case, the down-level client will view and match element type and attribute selectors based on their fully qualified name, not the local part as outlined in the Type selectors and Namespaces section. CSS selectors may be declared using an escaped colon "\:" to describe the fully qualified names, e.g. "html\:h1" will match <html:h1>. Selectors using the qualified name will only match XML elements that use the same prefix. Other namespace prefixes used in the XML that are mapped to the same URI will not match as expected unless additional CSS style rules are declared for them.
   • Note that selectors declared in this fashion will only match in down-level clients. A CSS namespace aware client will match element type and attribute selectors based on the name's local part. Selectors declared with the fully qualified name will not match (unless there is no namespace prefix in the fully qualified name).

In other scenarios: when the namespace prefixes used in the XML are not known in advance by the style sheet author; or a combination of elements with no namespace are used in conjunction with elements using a default namespace; or the same namespace prefix is mapped to different namespace URIs within the same document, or in different documents; it is impossible to construct a CSS style sheet that will function properly against all elements in those documents, unless, the style sheet is written using a namespace URI syntax (as outlined in this document or similar) and the document is processed by a CSS and XML namespace aware client.

12. Profiles

Each specification using Selectors must define the subset of W3C Selectors it allows and excludes, and describe the local meaning of all the components of that subset.

Non normative examples:

h1 a[name]

13. Conformance and requirements

This section defines conformance with the present specification only.

The inability of a user agent to implement part of this specification due to the limitations of a particular device (e.g., non interactive user agents will probably not implement dynamic pseudo-classes because they make no sense without interactivity) does not imply non-conformance.

All specifications reusing Selectors must contain a Profile listing the subset of Selectors it accepts or excludes, and describing the constraints it adds to the current specification.

Invalidity is caused by a parsing error, e.g. an unrecognized token or a token which is not allowed at the current parsing point.

User agents must observe the rules for handling parsing errors:

• a simple selector containing an undeclared namespace prefix is invalid
• a selector containing an invalid simple selector, an invalid combinator or an invalid token is invalid.
• a group of selectors containing an invalid selector is invalid.

Specifications reusing Selectors must define how to handle parsing errors. (In the case of CSS, the entire rule in which the selector is used is dropped.)

14. Tests

This specification has a test suite allowing user agents to verify their basic conformance to the specification. This test suite does not pretend to be exhaustive and does not cover all possible combined cases of Selectors.

15. Acknowledgements

The CSS working group would like to thank everyone who has sent comments on this specification over the years.

The working group would like to extend special thanks to Donna McManus, Justin Baker, Joel Sklar, and Molly Ives Brower who perfermed the final editorial review.

16. References

[CSS1]

Bert Bos, Håkon Wium Lie; "Cascading Style Sheets, level 1", W3C Recommendation, 17 Dec 1996, revised 11 Jan 1999

(http://www.w3.org/TR/REC-CSS1)

[CSS21]

Bert Bos, Tantek Çelik, Ian Hickson, Håkon Wium Lie, editors; "Cascading Style Sheets, level 2 revision 1", W3C Working Draft, 13 June 2005

(http://www.w3.org/TR/CSS21)

[CWWW]

Martin J. Dürst, François Yergeau, Misha Wolf, Asmus Freytag, Tex Texin, editors; "Character Model for the World Wide Web", W3C Recommendation, 15 February 2005

(http://www.w3.org/TR/charmod/)

[FLEX]

"Flex: The Lexical Scanner Generator", Version 2.3.7, ISBN 1882114213

[HTML4]

Dave Ragget, Arnaud Le Hors, Ian Jacobs, editors; "HTML 4.01 Specification", W3C Recommendation, 24 December 1999

(http://www.w3.org/TR/html4/)

[MATH]

Patrick Ion, Robert Miner, editors; "Mathematical Markup Language (MathML) 1.01", W3C Recommendation, revision of 7 July 1999

(http://www.w3.org/TR/REC-MathML/)

[RFC3066]

H. Alvestrand; "Tags for the Identification of Languages", Request for Comments 3066, January 2001

(http://www.ietf.org/rfc/rfc3066.txt)

[STTS]

Daniel Glazman; "Simple Tree Transformation Sheets 3", Electricité de France, submission to the W3C, 11 November 1998

(http://www.w3.org/TR/NOTE-STTS3)

[SVG]

Jon Ferraiolo, 藤沢 淳, Dean Jackson, editors; "Scalable Vector Graphics (SVG) 1.1 Specification", W3C Recommendation, 14 January 2003

(http://www.w3.org/TR/SVG/)

[UNICODE]

The Unicode Standard, Version 4.1, The Unicode Consortium. Boston, MA, Addison-Wesley, March 2005. ISBN 0-321-18578-1, as amended by Unicode 4.0.1 and Unicode 4.1.0.

(http://www.unicode.org/versions/)

[XML10]

Tim Bray, Jean Paoli, C. M. Sperberg-McQueen, Eve Maler, François Yergeau, editors; "Extensible Markup Language (XML) 1.0 (Third Edition)", W3C Recommendation, 4 February 2004

(http://www.w3.org/TR/REC-xml/)

[XMLNAMES]

Tim Bray, Dave Hollander, Andrew Layman, editors; "Namespaces in XML", W3C Recommendation, 14 January 1999

(http://www.w3.org/TR/REC-xml-names/)

[YACC]

S. C. Johnson; "YACC — Yet another compiler compiler", Technical Report, Murray Hill, 1975