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CSS | Cascading Style Sheets Examined

Cascading Style Sheets (CSS) is a style sheet language used to describe the presentation semantics (the look and formatting) 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 also be applied to any kind of XML document, including SVG and XUL.

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CSS is designed primarily to enable the separation of document content (written in HTML or a similar markup language) from document presentation, including elements such as the layout, colors, and fonts. This separation can improve content accessibility, provide more flexibility and control in the specification of presentation characteristics, enable multiple pages to share formatting, and reduce complexity and repetition in the structural content (such as by allowing for tableless web design). 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. While the author of a document typically links that document to a CSS style sheet, readers can use a different style sheet, perhaps one on their own computer, to override the one the author has specified.

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.

Summary

Style sheets are a very powerful tool for the Web site developer. They give you the chance to be completely consistent with the look and feel of your pages, while giving you much more control over the layout and design than straight HTML ever did.


Below is an example of an "Accordion" style menu effect done completely in CSS

Spring

Spring begins in the northern hemisphere around the months of March, April, May, or as reckoned astronomically extending from the vernal equinox. As in summer, the axis of the Earth is tilted toward the Sun and the length of daylight rapidly increases. The northern hemisphere begins to warm significantly causing new plant growth to "spring forth," giving the season its name. Snow begins to melt and streams swell with runoff and spring rains. Most flowering plants bloom this time of year, in a long succession sometimes beginning even when snow is still on the ground, and continuing into early summer. In normally snowless areas "spring" may begin as early as February during warmer years, with subtropical areas having very subtle differences, and tropical ones none at all. Subarctic areas may not experience "spring" at all until May or even June, or December in the outer Antarctic.

Severe weather most often occurs during the spring, when warm air begins to invade from lower latitudes while cold air is still pushing from the polar regions. Flooding is also most common in and near mountainous areas during this time of year because of snowmelt, many times accelerated by warm rains. In the United States, Tornado Alley is most active by far this time of year, especially since the Rocky Mountains prevent the surging hot and cold air masses from spreading eastward and instead force them directly at each other. Besides tornadoes, super cell thunderstorms can also produce dangerously large hail and very high winds, for which a severe thunderstorm warning or tornado warning is usually issued. Even more so than winter, the jet streams play an important role in severe weather in the springtime.

Summer

The seasons are considered by some Western countries to start at the equinoxes and solstices, based on astronomical reckoning. In North American-printed English-language calendars, based on astronomy, summer begins on the day of the summer solstice and ends on the day of the autumn equinox. When it is summer in the Southern Hemisphere, it is winter in the Northern Hemisphere, and vice versa.

But, because the seasonal lag is less than 1/8 of a year (except near large bodies of water), the meteorological start of the season, which is based on average temperature patterns, precedes by about three weeks the start of the astronomical season. According to meteorology, summer is the whole months of December, January, and February in the Southern Hemisphere, and the whole months of June, July, and August in the Northern Hemisphere. This meteorological definition of summer also aligns with commonly viewed notion of summer as the season with the longest (and warmest) days of the year, in which the daylight predominates, through varying degrees. The use of astronomical beginning of the seasons means that spring and summer have an almost equal pattern of the length of the days, with spring lengthening from the equinox to the solstice and summer shortening from the solstice to the equinox, while meteorological summer encompasses the build up to the longest day and decline thereafter, so that summer has many more hours of daylight than spring.

Today, the meteorological reckoning of the seasons is used in Australia, Denmark, the former USSR and by many people in the United Kingdom, but the astronomical definition is still more frequently used in the United States.

Autumn

Autumn (also known as fall in North American English) is one of the four temperate seasons. Autumn marks the transition from summer into winter. In the northern hemisphere, the start of autumn is generally considered to be around September, and in the southern hemisphere, its beginning is considered to be around March. There exist, however, different definitions of autumn, some of which are based on the months of the year while others are based on the equinox and solstice.

During autumn, deciduous trees shed their leaves. Leaves change to a yellowish, reddish or brownish hue before falling. Such coloured leaves have come to be colloquially called "fall foliage" in North America. In temperate zones, autumn is the season during which most crops are harvested. It is also the season during which days get shorter and cooler and the nights get longer.

Winter

Winter is one of the four seasons of temperate zones. North American calendars go by astronomy and state that winter begins on the winter solstice and ends on the vernal equinox. Calculated meteorologically, it begins and ends earlier (typically at the start of the month with the equinox or solstice) and is the season with the shortest days and the lowest temperatures. Either way, it generally has cold weather and, especially in the higher latitudes or altitudes, snow and ice. The coldest average temperatures of the season are typically experienced in January in the Northern Hemisphere and in July in the Southern Hemisphere.

Use

Prior to CSS, nearly all of the presentational attributes of 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 style sheet 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. The additional presentational markup in the HTML made documents more complex, and generally more difficult to maintain. In CSS, 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.

Sources

CSS information can be provided by various sources. CSS style information can be either attached as a separate document or embedded in the HTML document. Multiple style sheets can be imported. Different styles can be applied depending on the output device being used; for example, the screen version can be quite different from the printed version, so that authors can tailor the presentation appropriately for each medium.

Priority scheme for CSS sources (from highest to lowest priority):

  • Author styles (provided by the web page author), in the form of:
    • Inline styles, inside the HTML document, style information on a single element, specified using the "style" attribute.
    • Embedded style, blocks of CSS information inside the HTML itself.
    • External style sheets, i.e., a separate CSS file referenced from the document.
  • User style:
    • A local CSS file the user specifies with a browser option, which acts as an override applied to all documents.
  • User agent style:
    • Default styles applied by the user agent, i.e., the browser's default settings for element presentation.

The style sheet with the highest priority controls the content display. Declarations not set in the highest priority source are passed on by a source of lower priority such as the user agent style. This process is called cascading.

One of the goals of CSS is also to allow users greater control over presentation. Someone who finds red italic headings difficult to read may apply a different style sheet. Depending on their browser and the web site, a user may choose from various style sheets provided by the designers, may remove all added style and view the site using the browser's default styling, or may override just the red italic heading style without altering other attributes.

File highlightheaders.css containing:

h1 { color: white; background-color: orange !important; }
h2 { color: white; background-color: green !important; }

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.

History

Style sheets have existed 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 HTML grew, it came to encompass a wider variety of stylistic capabilities to meet the demands of web developers. This evolution gave the designer more control over site appearance but at the cost of HTML becoming more complex to write and maintain. Variations in web browser implementations i.e. ViolaWWW and WorldWideWeb made consistent site appearance difficult, and users had less control over how web content was displayed. Robert Cailliau wanted to separate the structure from the presentation. The ideal way would be to give the user different options and transferring three different kinds of style sheets: one for printing, one for the presentation on the screen and one for the editor feature.

To improve web presentation capabilities, nine different style sheet languages were proposed to the World Wide Web Consortium's (W3C) 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). CHSS, a language that has some resemblance to today's CSS, was proposed by Håkon Wium Lie in October 1994. Bert Bos was working on a browser called Argo, which used its own style sheet language the SSP. Lie and Yves Lafon joined Dave Raggett to expand the Arena browser for supporting CSS as a testbed application for the W3C.[5][6][7] Lie and Bos worked together to develop the CSS standard (the 'H' was removed from the name because these style sheets could also 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.

Lie's proposal was presented at the "Mosaic and the Web" conference (later called WWW2) in Chicago, Illinois in 1994, and again with Bert Bos in 1995. Around this time the W3C was already being established, and took an interest in the development of CSS. It 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). Lie and Bos were the primary technical staff on this aspect of the project, with additional members, including Thomas Reardon of Microsoft, participating as well. In August 1996 Netscape Communication Corporation presented an alternative style sheet language called JavaScript Style Sheets (JSSS). The spec was never finished and is deprecated. 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 2009.

In 2005 the CSS Working Groups decided to enforce the requirements for standards more strictly. This meant that already published standards like CSS 2.1, CSS 3 Selectors and CSS 3 Text were pulled back from Candidate Recommendation to Working Draft level.

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 was 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 2000, was the first browser to have full (better than 99 percent) CSS1 support[citation needed], surpassing Opera, which had been the leader since its introduction of CSS support 15 months earlier. Other browsers followed soon afterwards, and many of them additionally implemented parts of CSS2. As of August 2010[update], no (finished) browser has fully implemented CSS2, with implementation levels varying (see Comparison of layout engines (CSS)).

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 resorted to workarounds such as CSS hacks and CSS filters to obtain consistent results across web browsers and platforms.

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 moved nearer to a working snapshot of current CSS support in HTML browsers. Some CSS2 properties that no browser successfully implemented were dropped, and in a few cases, defined behaviors were changed to bring the standard into line with the predominant existing implementations. CSS2.1 became a Candidate Recommendation on February 25, 2004, but CSS2.1 was pulled back to Working Draft status on June 13, 2005, and only returned to Candidate Recommendation status on July 19, 2007.

In the past, some web servers were configured to serve all documents with the filename extension .css as mime type application/x-pointplus[13] rather than text/css. At the time, the Net-Scene company was selling PointPlus Maker to convert PowerPoint files into Compact Slide Show files (using a .css extension).

Variations

CSS has various levels and profiles. Each level of CSS builds upon the last, typically adding new features and 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.

  • CSS 1

    The first CSS specification to become an official W3C Recommendation is CSS level 1, published in December 1996. Among its capabilities are support for:

    • Font properties such as typeface and emphasis
    • Color of text, backgrounds, and other elements
    • Text attributes such as spacing between words, letters, and lines of text
    • Alignment of text, images, tables and other elements
    • Margin, border, padding, and positioning for most elements
    • Unique identification and generic classification of groups of attributes

    Note: The W3C no longer maintains the CSS1 Recommendation.

  • CSS 2

    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 and z-index, 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 June 15, 2005 it was reverted to a working draft for further review.[18] It was returned to Candidate Recommendation status on 19 July 2007.

  • CSS 3

    CSS level 3 has been under development since December 15, 2005. The W3C maintains a CSS3 progress report. CSS3 is modularized and consists of several separate recommendations. The W3C CSS3 Roadmap provides a summary and introduction.

Browser Support

Further information: Comparison of layout engines (Cascading Style Sheets)

Because not all browsers comply identically with CSS code, a coding technique known as a CSS filter can be used to show or hide 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 different CSS to certain browsers to ensure designs render 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. Internet Explorer support for CSS began with IE 3.0 and increased progressively with each version. By 2008, the first Beta of Internet Explorer 8 offered support for CSS 2.1 in its best web standards mode.

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 that 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 CSS 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 Netscape, Mozilla Firefox, Opera, and Internet Explorer (including Internet Explorer 7).

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 was intended to 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 used in Firefox, the WebKit layout engine used in Apple Safari and Google Chrome, the similar KHTML engine used in KDE's Konqueror browser, and Opera's Presto layout engine—each of them is leading in different aspects of CSS. As of August 2009, Internet Explorer 8, Firefox 2 and 3 have reasonably complete levels of implementation of CSS 2.1.

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