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