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In computing, Cascading Style Sheets (CSS) is a stylesheet language used to describe the presentation of a document written in a markup language. Its most common application is to style web pages written in HTML and XHTML, but the language can be applied to any kind of XML document, including SVG and XUL. The CSS specifications are maintained by the World Wide Web Consortium (W3C). CSS has various levels and profiles. Each level of CSS builds upon the last, typically adding new features and are typically denoted as CSS1, CSS2, and CSS3. Profiles are typically a subset of one or more levels of CSS built for a particular device or user interface. Currently there are profiles for mobile devices, printers, and television sets. Profiles should not be confused with media types which were added in CSS2. The use of CSS to position the content of a web page is sometimes referred to as CSS-P or CSS Positioning . Overview CSS is used by both the authors and readers of web pages to define colors, fonts, layout, and other aspects of document presentation. It is designed primarily to enable the separation of document content (written in HTML or a similar markup language) from document presentation (written in CSS). This separation can improve content accessibility, provide more flexibility and control in the specification of presentational characteristics, and reduce complexity and repetition in the structural content. CSS can also allow the same markup page to be presented in different styles for different rendering methods, such as on-screen, in print, by voice (when read out by a speech-based browser or screen reader) and on braille-based, tactile devices. Similarly, identical HTML or XML markup can be displayed in a variety of styles, 'brands', liveries or colour schemes by using different CSS. CSS information can be provided by various sources: CSS specifies a priority scheme to determine which style rules apply if more than one rule matches against a particular element. In this so-called 'cascade', priorities or 'weights' are calculated and assigned to rules, so that the results are predictable. Advantages of using CSS include: CSS has a simple syntax, and uses a number of English keywords to specify the names of various style properties. A style sheet consists of a list of rules. Each rule consists of one or more comma-separated selectors and a declaration block. A declaration-block consists of a list of semicolon-separated declarations in curly braces. Each declaration itself consists of a property, a colon (:) then a value. Example: p h2 .note p.warning These are seven rules, with selectors p, h2, .note, p.warning, a:hover and Property values are specified by, for example, color: red, where the property color is given the value red.In the first two rules, the HTML elements p (paragraph) and h2 (level-two heading) are being assigned stylistic attributes. The paragraph element will be rendered in Garamond font or, if Garamond is unavailable, some other serif font. The level-two heading element will be rendered in red on a white background.The third rule shown matches those elements that have a class attribute containing the token 'note'. For example:The fourth rule shown matches those p elements that have a class attribute containing the token 'warning'. This is in contrast to the third rule which matched all elements, not just paragraph tags, that are marked with a given class attribute (the third rule .note could also have been written as In fact, .class selectors involve a special kind of attribute matching, as opposed to testing for equality. Since the HTML class attribute is defined as a whitespace-separated list of tokens, a class selector is evaluated against each of them separately. For example, note and the warning rule.The fifth rule will match the one and only element in each HTML document whose id attribute is set to paragraph1: It will have no margin within its rendering 'box' as its margin width is set to zero.The sixth rule defines the hover style for a elements. By default in most browsers, a elements are underlined. This rule will remove the underline when the user "hovers" the mouse cursor over these elements.The last rule matches those p elements that are within the document element whose id attribute has the value news. It is an example of a descendant selector.Because of the cascading nature of CSS rules, these last p elements will inherit font-family: "Garamond", serif as with all p elements. Those that have the token warning in the value of their class attribute, will inherit that background image too.A CSS stylesheet can contain comments; the syntax for comments is similar to that used in the C programming language /
In CSS, selectors are used to declare which elements a style applies to, a kind of match expression. Here are some examples of selector syntax: All elementsthat is, using the By element namee.g. for all Descendantse.g. for class or id attributese.g. Adjacent elementse.g. for all Direct child elemente.g. for all By attributee.g. for all In addition to these, a set of pseudo-classes can be used to define further behavior. Probably the best-known of these is :hover, which applies a style only when the user 'points to' the visible element, usually by holding the mouse cursor over it. It is appended to a selector as in a:hover or :first-line, :visited or :before. A special pseudo-class is :lang(c), where the style would be applied on an element only if it is in language "c".
Selectors may be combined in other ways too, especially in CSS 2.1, to achieve greater specificity and flexibility (see the complete definition of selectors at the W3C Web site). In the following example, several selectors are grouped using comma separation*. The rule sets the font for all HTML headings. h1, h2, h3, h4, h5, h6 To use a CSS stylesheet, save the CSS code in a file such as example.css and then either link to it or import it from HTML or XHTML web pages using one of the two following formats: In the first example, note that the /> syntax only applies in XHTML; If you are writing HTML, close an empty element such as this with >.CSS styles may also be specified in the style attribute.To apply a CSS stylesheet to an XML document, add the following processing instruction, as per the XML example below: History Style sheets have been around in one form or another since the beginnings of SGML in the 1970s. Cascading Style Sheets were developed as a means for creating a consistent approach to providing style information for web documents. As the HTML grew, it came to encompass a wider variety of stylistic capabilities to meet the demands of web developers. This gave the designer more control over site appearance, but HTML became more complex to write and maintain. Variations in web browser implementations made consistent site appearance difficult and users had less control over their web browsing experience. To improve the capabilities of web presentation, nine different style sheet languages were proposed to the W3C's www-style mailing list. Of the nine proposals, two were chosen as the foundation for what became CSS: Cascading HTML Style Sheets (CHSS) and Stream-based Style Sheet Proposal (SSP). Firstly, Håkon Wium Lie (now the CTO of Opera Software) proposed Cascading HTML Style Sheets (CHSS) in October 1994, a language which has some resemblance to today's CSS. Bert Bos was working on a browser called Argo which used its own style sheet language, Stream-based Style Sheet Proposal (SSP). Lie and Bos worked together to develop the CSS standard (the 'H' was removed from the name because these style sheets could be applied to other markup languages besides HTML). Unlike existing style languages like DSSSL and FOSI, CSS allowed a document's style to be influenced by multiple style sheets. One style sheet could inherit or "cascade" from another, permitting a mixture of stylistic preferences controlled equally by the site designer and user. Håkon's proposal was presented at the "Mosaic and the Web" conference in Chicago, Illinois in 1994, and again with Bert Bos in 1995. Around this time, the World Wide Web Consortium was being established; the W3C took an interest in the development of CSS, and organized a workshop toward that end chaired by Steven Pemberton. This resulted in W3C adding work on CSS to the deliverables of the HTML editorial review board (ERB). Håkon and Bert were the primary technical staff on this aspect of the project, with additional members, including Thomas Reardon of Microsoft, participating as well. By the end of 1996, CSS was ready to become official and the CSS level 1 Recommendation was published in December. Development of HTML, CSS, and the DOM had all been taking place in one group, the HTML Editorial Review Board (ERB). Early in 1997, the ERB was split into three working groups: HTML Working group, chaired by Dan Connolly of W3C, DOM Working group, chaired by Lauren Wood of SoftQuad, and CSS Working group, chaired by Chris Lilley of W3C. The CSS Working Group began tackling issues that had not been addressed with CSS level 1, resulting in the creation of CSS level 2 on November 4 1997. It was published as a W3C Recommendation on May 12 1998. CSS level 3, which was started in 1998, is still under development as of 2006. Difficulty with adoption Although the CSS1 specification was completed in 1996 and Microsoft's Internet Explorer 3 was released in that year featuring some limited support for CSS, it would be more than three years before any web browser achieved near-full implementation of the specification. Internet Explorer 5.0 for the Macintosh, shipped in March of 2000, was the first browser to have full (better than 99 percent) CSS1 support, surpassing Opera, which had been the leader since its introduction of CSS support fifteen months earlier. Other browsers followed soon afterwards, and many of them additionally implemented parts of CSS2. As of July 2006, no browser has fully implemented CSS2, with implementation levels varying (see Comparison of layout engines (CSS)). Although the exact levels of conformance are not easy to judge, it is generally accepted that Internet Explorer has the worst CSS implementation of all modern browsers (excluding user agents which do not implement CSS, e.g., Lynx). Even though early browsers such as Internet Explorer 3 and 4, and Netscape 4.x had support for CSS, it was typically incomplete and afflicted with serious bugs. This was a serious obstacle for the adoption of CSS. When later 'version 5' browsers began to offer a fairly full implementation of CSS, they were still incorrect in certain areas and were fraught with inconsistencies, bugs and other quirks. The proliferation of such CSS-related inconsistencies and even the variation in feature support has made it difficult for designers to achieve a consistent appearance across platforms. Some authors commonly resort to using CSS hacks, workarounds, and CSS filters in order to obtain consistent results across web browsers and platforms. A 'CSS filter' is a coding technique that aims to effectively hide or show parts of the CSS to different browsers, either by exploiting CSS-handling quirks or bugs in the browser, or by taking advantage of lack of support for parts of the CSS specifications. Using CSS filters, some designers have gone as far as delivering entirely different CSS to certain browsers in order to ensure that designs are rendered as expected. Because very early web browsers were either completely incapable of handling CSS, or render CSS very poorly, designers today often routinely use CSS filters that completely prevent these browsers from accessing any of the CSS. An example of a well-known CSS browser bug is the Internet Explorer box model bug, where box widths are interpreted incorrectly in several versions of the browser, resulting in blocks which are too narrow when viewed in Internet Explorer, but correct in standards-compliant browsers. The bug can be avoided in Internet Explorer 6 by using the correct doctype in (X)HTML documents. CSS hacks and filters are used to compensate for bugs such as this, just one of hundreds of CSS bugs that have been documented in various versions of Internet Explorer, Netscape, Mozilla, and Opera. * Even when the availability of CSS-capable browsers made CSS a viable technology, the adoption of CSS was still held back by designers' struggles with browsers' incorrect CSS implementation and patchy CSS support. Even today, these problems continue to make the business of CSS design more complex and costly than it should be, and cross-browser testing remains a necessity. Other reasons for continuing non-adoption of CSS are: its perceived complexity, authors' lack of familiarity with CSS syntax and required techniques, poor support from authoring tools, the risks posed by inconsistency between browsers and the increased costs of testing. Currently there is strong competition between Mozilla's Gecko layout engine, Opera's Presto layout engine, and the KHTML engine used in both Apple's Safari and KDE's Konqueror browsers - each of them is leading in different aspects of CSS. As of 2006, Internet Explorer remains the worst at rendering CSS as judged by World Wide Web Consortium standards (* as linked from *). Problems with browsers' patchy adoption of CSS along with errata in the original specification led the W3C to revise the CSS2 standard into CSS2.1, which may be regarded as something nearer to a working snapshot of current CSS support in HTML browsers. Some CSS2 properties which no browser had successfully implemented were dropped, and in a few cases, defined behaviours were changed to bring the standard into line with the predominant existing implementations. CSS2.1 became a Candidate Recommendation on February 25, 2004; but was pulled back to Working Draft status on June 13, 2005 to fix various issues (in some cases, to match more closely to browser implementation). Internet media type Internet media type (MIME type) text/css is registered for use with CSS by RFC 2318 (March 1998).As of 2006 some older web servers are still configured to serve documents with the filename extension .css as mime type application/x-pointplus. This is because the Net-Scene company was selling PointPlus Maker to convert PowerPoint files into Compact Slide Show files (using the .css extension) and web servers were configured to signal to client browsers that these .css files were x-pointplus media type. Since the plugin was listed in the directory for Netscape Navigator 3.0, the popular Netscape Enterprise Server was distributed with this mapping pre-configured. When reading external style sheets some web browsers try to compensate for the misconfigured web servers by treating the PointPlus media type as a text/css media type instead, but some (notably Firefox) comply with the media type and will not render the external CSS file as a style sheet.Use of CSS Prior to CSS, nearly all of the presentational attributes of a HTML documents were contained within the HTML markup; all font colors, background styles, element alignments, borders and sizes had to be explicitly described, often repeatedly, within the HTML. CSS allows authors to move much of that information to a separate stylesheet resulting in considerably simpler HTML markup. Headings ( h1 elements), sub-headings (h2), sub-sub-headings (h3) etc. are defined structurally using HTML. In print and on the screen, choice of font, size, color and emphasis for these elements is presentational.Prior to CSS, document authors who wanted to assign such typographic characteristics to, say, all h2 headings had to use the HTML font and other presentational elements for each occurrence of that heading type. A heading to be centred on the page in italic, red, Times New Roman might be written:The additional presentational markup in the HTML made documents more complex, and generally more difficult to maintain. To render all h2 tags in this manner, the markup had to be repeated for each heading. With CSS, the h2 and other elements structure the document, while the style sheet defines presentational characteristics. The above might be written:While the following block in an accompanying style sheet defines the same style for all default h2 headings across the web site:h2 Thus, presentation is separated from structure. In print, CSS can define color, font, text alignment, size, borders, spacing, layout and many other typographic characteristics. It can do so independently for on-screen and printed views. CSS also defines non-visual styles such as the speed and emphasis with which text is read out by aural text readers. The W3C now considers the advantages of CSS for defining all aspects of the presentation of HTML pages to be superior to other methods. It has therefore deprecated the use of all the original presentational HTML markup. CSS style information can be either attached as a separate document or embedded in the HTML document. Multiple style sheets can be imported, and alternative style sheets can be specified so that the user can choose between them. Different styles can be applied depending on the output device being used as in the screen version being quite different to the printed version so that authors can tailor the presentation appropriately for each medium. One of the goals of CSS is also to allow users a greater degree of control over presentation; those who find the red italic headings difficult to read may apply other style sheets to the document. Depending on their browser and the web site, a user may choose from various stylesheets provided by the designers, may remove all added style and view the site using their browser's default styling or may perhaps override just the red italic heading style without altering other attributes. See also presentational markup, which gives example CSS code, along with the deprecated tags. Example of an XHTML document utilizing CSS
Example of a user style sheet File highlightheaders.css containing: h1 h2 Such a file is stored locally and is applicable if that has been specified in the browser options. "!important" means that it prevails over the author specifications. Example of applying CSS to plain XML
CSS limitations Most problems attributed to CSS are actually results of browser bugs or lack of support for CSS features. The most serious offender among current browsers is Microsoft Internet Explorer, whose version 6 lacks support for about 30 percent of CSS2 properties, and, more significantly, misinterprets a significant number of important properties, such as " width", "height", and "float".However, current CSS specifications do have some genuine shortcomings. Selectors are unable to ascendCSS offers no way to select a parent or ancestor of element that satisfies certain criteria. A more advanced selector scheme (such as XPath) would enable more sophisticated stylesheets. However, the major reasons for the CSS Working Group rejecting proposals for parent selectors are related to browser performance and incremental rendering issues. One block declaration cannot explicitly inherit from anotherInheritance of styles is performed by the browser based on the containment hierarchy of DOM elements and the specificity of the rule selectors, as suggested by the section 6.4.1 of the CSS2 specification *. Only the user of the blocks can refer to them by including class names into the Vertical control limitationsWhile horizontal placement of elements is generally easy to control, vertical placement is frequently unintuitive, convoluted, or impossible. Simple tasks, such as centering an element vertically or getting a footer to be placed no higher than bottom of viewport, either require complicated and unintuitive style rules, or simple but widely unsupported rules. Absence of expressionsThere is currently no ability to specify property values as simple expressions (such as Lack of orthogonalityMultiple properties often end up doing the same job. For instance, Unexpected margin collapsingMargin collapsing is frequently not expected by authors, and no simple side-effect-free way is available to control it. Float containmentCSS does not explicitly offer any property that would force an element to contain floats. Multiple properties offer this functionality as a side effect, but none of them are completely appropriate in all situations. Lack of multiple backgrounds per elementHighly graphical designs require several background images for every element, and CSS can support only one. Therefore, developers have to choose between adding redundant wrappers around document elements, or dropping the visual effect. This is partially addressed in the working draft of the CSS3 backgrounds module,*, which is already supported Safari, but implementations in other browsers are likely to be several years away. Recommendations The first CSS specification to become an official W3C Recommendation is CSS level 1, published in December 1996. Among its capabilities are support for: The W3C maintains the CSS1 Recommendation. CSS level 2 was developed by the W3C and published as a Recommendation in May 1998. A superset of CSS1, CSS2 includes a number of new capabilities like absolute, relative, and fixed positioning of elements, the concept of media types, support for aural style sheets and bidirectional text, and new font properties such as shadows. The W3C maintains the CSS2 Recommendation. CSS level 2 revision 1 or CSS 2.1 fixes errors in CSS2, removes poorly-supported features and adds already-implemented browser extensions to the specification. While it was a Candidate Recommendation for several months, on 15 June 2005 it was reverted to a working draft for further review. CSS level 3 is currently under development. The W3C maintains a CSS3 progress report. As it did with the evolving XHTML specification, CSS3 was modularized and will consist of several separate Recommendations. An Introduction to CSS3 roadmap will be the starting point. See also Further reading | |||||||||
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