CSS

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, tactileHTML or XML markup can be displayed in a variety of styles, liveries or color schemes by using different CSS. devices. Similarly, identical

CSS information can be provided by various sources:

• Author styles (style information provided by the web page author), in the form of
  • external stylesheets, i.e. a separate CSS-file referenced from the document
  • embedded style, blocks of CSS information inside the HTML document itself
  • inline styles, inside the HTML document, style information on a single element, specified using the "style" attribute.
• User style
  • a local CSS-file specified by the user using options in the web browser, and acting as an override, to be applied to all documents.
• User agent style
  • the default style sheet applied by the user agent, e.g. the browser's default presentation of elements.

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

Advantages of using CSS include:

• Presentation information for an entire website or collection of pages can be held in one CSS file, allowing sweeping changes to be propagated with quick changes to this one file.
• Web browser software can store a copy of the CSS file in the computers cache, so that it doesn't have to be downloaded each time the user views a web page that references it.
• Different users can have different style sheets: for example a large text alternative for visually-impaired users, or a layout optimized for small displays for mobile phones.
• The document code is reduced in size and complexity, since it does not need to contain any presentational markup.

Use of CSS

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 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.