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1 About the CSS2 Specification

Contents

1.1 Reading the specification

This specification has been written with two types of readers in mind: CSS authors and CSS implementors. We hope the specification will provide authors with the tools they need to write efficient, attractive, and accessible documents, without overexposing them to CSS's implementation details. Implementors, however, should find all they need to build conforming user agents. The specification begins with a general presentation of CSS and becomes more and more technical and specific towards the end. For quick access to information, a general table of contents, specific tables of contents at the beginning of each section, and an index provide easy navigation, in both the electronic and printed versions.

The specification has been written with two modes of presentation in mind: electronic and printed. Although the two presentations will no doubt be similar, readers will find some differences. For example, links will not work in the printed version (obviously), and page numbers will not appear in the electronic version. In case of a discrepancy, the electronic version is considered the authoritative version of the document.

1.2 How the specification is organized

The specification is organized into the following sections:

Section 2: An introduction to CSS2
The introduction includes a brief tutorial on CSS2 and a discussion of design principles behind CSS2.
Sections 3 - 20: CSS2 reference manual.
The bulk of the reference manual consists of the CSS2 language reference. This reference defines what may go into a CSS2 style sheet (syntax, properties, property values) and how user agents must interpret these style sheets in order to claim conformance.
Appendixes:
Appendixes contain information about a sample style sheet for HTML 4.0, changes from CSS1 , implementation and performance notes, the grammar of CSS2, a list of normative and informative references, and three indexes: one for properties, one for descriptors, and one general index.

1.3 Conventions

1.3.1 Document language elements and attributes

1.3.2 CSS property definitions

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

'property-name'
Value:  legal values & syntax
Initial:  initial value
Applies to:  elements this property applies to
Inherited:  whether the property is inherited
Percentages:  how percentage values are interpreted
Media:  which media groups the property applies to

Value

This part specifies the set of valid values for the property. Value types may be 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'>). 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.

Values may be arranged as follows:

Juxtaposition 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
  [ a b ] | [ c || [ d e ]]

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

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>

Initial

This part specifies the property's initial value. If the property is inherited, this is the value that is given to the root element of the document tree. Please consult the section on the cascade for information about the interaction between style sheet-specified, inherited, and initial values.

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, 'white-space' only affects block-level elements.

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 values.

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 as values.

Media groups

This part indicates the media groups to which the property applies. The conformance conditions state that user agents must support this property if they support rendering to the media types included in these media groups.

1.3.3 Shorthand properties

Some properties are shorthand properties, meaning 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).

Example(s):

The multiple style rules of this example:

H1 { 
  font-weight: bold; 
  font-size: 12pt;
  line-height: 14pt; 
  font-family: Helvetica; 
  font-variant: normal;
  font-style: normal;
  font-stretch: normal;
  font-size-adjust: none
}

may be rewritten with a single shorthand property:

H1 { font: bold 12pt/14pt Helvetica }

In this example, 'font-variant', 'font-stretch', 'font-size-adjust', and 'font-style' take their initial values.

1.3.4 Notes and examples

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

All HTML examples conform to the HTML 4.0 strict DTD (defined in [HTML40]) unless otherwise indicated by a document type declaration.

All notes are informative only.

Examples and notes are marked within the source HTML for the specification and CSS1 user agents will render them specially.

1.3.5 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]" to the right of the image.

Images and long descriptions are informative only.

1.4 Acknowledgments

This specification is the product of the W3C Working Group on Cascading Style Sheets and Formatting Properties. In addition to the editors of this specification, the members of the Working Group are: Brad Chase (Bitstream), Chris Wilson (Microsoft), Daniel Glazman (Electricité de France), Dave Raggett (W3C/HP), Ed Tecot (Microsoft), Jared Sorensen (Novell), Lauren Wood (SoftQuad), Laurie Anna Kaplan (Microsoft), Mike Wexler (Adobe), Murray Maloney (Grif), Powell Smith (IBM), Robert Stevahn (HP), Steve Byrne (JavaSoft), Steven Pemberton (CWI), Thom Phillabaum (Netscape), Douglas Rand (Silicon Graphics), Robert Pernett (Lotus), Dwayne Dicks (SoftQuad), and Sho Kuwamoto (Macromedia). We thank them for their continued efforts.

A number of invited experts to the Working Group have contributed: George Kersher, Glenn Rippel (Bitstream), Jeff Veen (HotWired), Markku T. Hakkinen (The Productivity Works), Martin Dürst (W3C, formerly Universität Zürich), Roy Platon (RAL), Todd Fahrner (Verso), Tim Boland (NIST), Eric Meyer (Case Western Reserve University), and Vincent Quint (W3C).

The section on Web Fonts was strongly shaped by Brad Chase (Bitstream) David Meltzer (Microsoft Typography) and Steve Zilles (Adobe). The following people have also contributed in various ways to the section pertaining to fonts: Alex Beamon (Apple), Ashok Saxena (Adobe), Ben Bauermeister (HP), Dave Raggett (W3C/HP), David Opstad (Apple), David Goldsmith (Apple), Ed Tecot (Microsoft), Erik van Blokland (LettError), François Yergeau (Alis), Gavin Nicol (Inso), Herbert van Zijl (Elsevier), Liam Quin, Misha Wolf (Reuters), Paul Haeberli (SGI), and the late Phil Karlton (Netscape).

The section on Paged Media was in large parts authored by Robert Stevahn (HP) and Stephen Waters (Microsoft).

Robert Stevahn (HP), Scott Furman (Netscape), and Scott Isaacs (Microsoft) were key contributors to CSS Positioning.

Mike Wexler (Adobe) was the editor of the interim working draft, which described many of the new features of CSS2.

T.V. Raman (Adobe) made pivotal contributions towards Aural Cascading Style Sheets (ACSS) and the concepts of aural presentation based on his work on AsTeR (Audio System For Technical Readings). He contributed an initial draft of the ACSS specification that shaped the current specification. Values for aural properties in the HTML 4.0 sample style sheet are of his devising; he currently uses them on a daily basis on his audio desktop in conjunction with Emacspeak and the Emacs W3 browser (authored by William Perry, who also implemented the aural extensions on the W3 side of the fence).

Todd Fahrner (Verso) researched contemporary and historical browsers to develop the sample style sheet in the appendix.

Thanks to Jan Kärrman, author of html2ps for helping so much in creating the PostScript version of the specification.

Through electronic and physical encounters, the following people have contributed to the development of CSS2: Alan Borning, Robert Cailliau, Liz Castro, James Clark, Dan Connolly, Donna Converse, Daniel Dardailler, Al Gilman, Daniel Greene, Scott Isaacs, Geir Ivarsøy, Vincent Mallet, Kim Marriott, Brian Michalowski, Lou Montulli, Henrik Frystyk Nielsen, Jacob Nielsen, Eva von Pepel, William Perry, David Siegel, Peter Stuckey, and Jason White.

The discussions on www-style@w3.org have been influential in many key issues for CSS. Especially, we would like to thank Bjorn Backlund, Todd Fahrner, Lars Marius Garshol, Sue Jordan, Ian Hickson, Susan Lesch, Andrew Marshall, MegaZone, Eric Meyer, Russell O'Connor, David Perrell, Liam Quinn, Jon Seymour, Neil St. Laurent, Taylor, Brian Wilson, and Chris Wilson for their participation.

Many thanks to the Web Accessibility Initiative Protocols and Formats Technical Review Working Group (WAI PF) for helping to improve the accessibility of CSS2.

Many thanks to Philippe Le Hégaret, whose CSS validator helped ensure correct examples and a sensible grammar.

Special thanks to Arnaud Le Hors, whose engineering contributions made this document work.

Adam Costello improved this specification by performing a detailed review.

Lastly, thanks to Tim Berners-Lee without whom none of this would have been possible.

1.5 Copyright Notice

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