1 About the CSS2CSS 2.1 Specification


1.1 CSS 2.1 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 have subsequently been corrected via the publication of various errata, but there has not yet been an opportunity for the specification to be changed based on experience gained.

While many of these issues will be addressed by the upcoming CSS3 specifications, the current state of affairs hinders the implementation and interoperability of CSS2. The CSS 2.1 specification attempts to address this situation by:

Thus, while it is not the case that a CSS2 style sheet is necessarily forwards-compatible with CSS 2.1, it is the case that a style sheet restricting itself to CSS 2.1 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 2.1 are worthwhile.

CSS 2.1 is derived from and is intended to replace CSS2. Some parts of CSS2 are unchanged in CSS 2.1, 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 2.1 (unless they need features from CSS2 which have been dropped in CSS 2.1, 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)).

1.2 Reading the specification

This section is non-normative.

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.

1.21.3 How the specification is organized

This section is non-normative.

The specification is organized into the following sections:

Section 2: An introduction to CSS2CSS 2.1
The introduction includes a brief tutorial on CSS2CSS 2.1 and a discussion of design principles behind CSS2.CSS 2.1.
Sections 3 - 20: CSS2CSS 2.1 reference manual.
The bulk of the reference manual consists of the CSS2CSS 2.1 language reference. This reference defines what may go into a CSS2CSS 2.1 style sheet (syntax, properties, property values) and how user agents must interpret these style sheets in order to claim conformance.
Appendixes contain information about aural properties (non-normative), a sample style sheet for HTML 4.04, changes from CSS1 , implementation and performance notesCSS2, the grammar of CSS2CSS 2.1, a list of normative and informative references, and threetwo indexes: one for properties , one for descriptors , andand one general index.

1.31.4 Conventions Document language elements and attributes CSS property definitions

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

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
Computed value:  how to compute the computed value Value

This part specifies the set of valid values for the property.property whose name is 'property-name'. 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'>). 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.

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>

Value types 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 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'. 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'the 'clear' property 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 theseInformation about media groups . 1.3.3is non-normative. 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).


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. Notes and examples

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

AllHTML examples conformlacking DOCTYPE declarations are SGML Text Entities conforming to the HTML 4.04.01 Strict DTD (defined[HTML4]. Other HTML examples conform to the DTDs given in [HTML40] ) unless otherwise indicated by a document type declaration.the examples.

All notes are informative only.

Examples and notes are marked within the source HTML for the specification and CSS1CSS user agents will render them specially. 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 ofafter the image.

Images and long descriptions are informative only.

1.41.5 Acknowledgments

This specificationsection is the product of the W3C Working Groupnon-normative.

CSS 2.1 is based on Cascading Style Sheets and Formatting Properties. In addition to the editors of this specification,CSS2. See the membersacknowledgments section 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 themCSS2 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 followingpeople have alsothat contributed in various waysto the section pertainingCSS2.

We would like 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).thank the section on Paged Media was in large parts authored by Robert Stevahn (HP) and Stephen Waters (Microsoft). Robert Stevahn (HP), Scott Furman (Netscape),following people who, through their input and Scott Isaacs (Microsoft) were key contributors to CSS Positioning. Mike Wexler (Adobe) wasfeedback on the editor ofwww-style mailing list, have helped us with the interim working draft, which described manycreation 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,this specification: Andrew Clover, Bernd Mielke, C. Bottelier, Christian Roth, Christoph Päper, Claus Färber, Coises, Craig Saila, Darren Ferguson, Dylan Schiemann, Etan Wexler, George Lund, 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,Craig, Jan Eirik Olufsen, Jan Roland Eriksson, Joris Huizer, Joshua Prowse, Kai Lahmann, Kevin Smith, Lachlan Cannon, Lars Knoll, Lauri Raittila, Mark Gallagher, Michael Day, Peter Stuckey,Sheerin, Rijk van Geijtenbeek, Robin Berjon, Scott Montgomery, Shelby Moore, Stuart Ballard, Tom Gilder, Vadim Plessky, and Jason White.the discussions on www-style@w3.org have been influential in many key issues for CSS. Especially,Open eBook Publication Structure Working Group Editors. We would also 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 ProtocolsGlenn Adams and Formats Technical Review Working Group (WAI PF) for helping to improve the accessibility of CSS2. Many thanks to Philippe Le Hégaret, whose CSS validatorSusan Lesch who 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 Copyright © 1997 World Wide Web Consortium , ( Massachusetts Institute of Technology , Institut National de Recherche en Informatique et en Automatique , Keio University ). All Rights Reserved. Documents on the W3C site are provided by the copyright holders under the following license. By obtaining, using and/or copying this document, or the W3C document from which this statement is linked, you agree that you have read, understood, and will comply with the following terms and conditions: Permission to use, copy, and distribute the contents ofproofread this document, or the W3C document from which this statement is linked, in any medium for any purpose and without fee or royalty is hereby granted, provided that you include the following on ALL copies of the document, or portions thereof, that you use: A link or URI to the original W3C document. The pre-existing copyright notice of the original author, if it doesn't exist, a notice of the form: "Copyright © World Wide Web Consortium , ( Massachusetts Institute of Technology , Institut National de Recherche en Informatique et en Automatique , Keio University ). All Rights Reserved." If it exists , the STATUS of the W3Cdocument.

When space permits, inclusion of the full text of this NOTICE should be provided.In addition, credit shall be attributed to the copyright holders for any software, documents, or other items or products that you create pursuant to the implementation of the contents of this document, or any portion thereof. No right to create modifications or derivatives is granted pursuantwe would like to this license. THIS DOCUMENT IS PROVIDED "AS IS," AND COPYRIGHT HOLDERS MAKE NO REPRESENTATIONS OR WARRANTIES, EXPRESS OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, NON-INFRINGEMENT, OR TITLE; THAT THE CONTENTS OF THE DOCUMENT ARE SUITABLE FOR ANY PURPOSE; NOR THAT THE IMPLEMENTATION OF SUCH CONTENTS WILL NOT INFRINGE ANY THIRD PARTY PATENTS, COPYRIGHTS, TRADEMARKS OR OTHER RIGHTS. COPYRIGHT HOLDERS WILL NOT BE LIABLE FOR ANY DIRECT, INDIRECT,extend special OR CONSEQUENTIAL DAMAGES ARISING OUT OF ANY USE OF THE DOCUMENT OR THE PERFORMANCE OR IMPLEMENTATION OF THE CONTENTS THEREOF. The namethanks to fantasai, Ada Chan and trademarks of copyright holders may NOT be used in advertising or publicity pertainingBoris Zbarsky who have contributed significant time to this document or its contents without specific, written prior permission. TitleCSS 2.1, and to copyright in this document will at all times remainKimberly Blessing for help with copyright holders.the editing.