From Wikipedia, the free encyclopedia
Cascading Style Sheets (CSS) is a style sheet language used for
describing the presentation of a document written in a markup language
such as HTML.[1] CSS is a cornerstone technology of the World Wide Web,
alongside HTML and JavaScript.[2]
CSS is designed to enable the separation
of presentation and content, including layout, colors, and fonts.[3] This
separation can improve content accessibility, provide more flexibility and
control in the specification of presentation characteristics, enable
multiple web pages to share formatting by specifying the relevant CSS in a
separate .css file which reduces complexity and repetition in the
structural content as well as enabling the .css file to be cached to
improve the page load speed between the pages that share the file and its
formatting. Separation of formatting and content also makes it feasible to
present the same markup page in different styles for different rendering
methods, such as on-screen, in print, by voice (via speech-based browser
or screen reader), and on Braille-based tactile devices. CSS also has
rules for alternate formatting if the content is accessed on a mobile
device.[4] The name cascading comes from the specified priority scheme to
determine which style rule applies if more than one rule matches a
particular element. This cascading priority scheme is predictable. The CSS
specifications are maintained by the World Wide Web Consortium (W3C).
Internet media type (MIME type) text/css is registered for use with CSS by
RFC 2318 (March 1998). The W3C operates a free CSS validation service for
CSS documents.[5] In addition to HTML, other markup languages support the
use of CSS including XHTML, plain XML, SVG, and XUL.
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 or rule-set consists of one or more selectors, and a declaration block.
In CSS, selectors declare which part of the markup a style applies to by matching tags and attributes in the markup itself. Selectors may apply to the following:
Classes and IDs are case-sensitive, start with letters, and can include alphanumeric characters, hyphens, and underscores. A class may apply to any number of instances of any elements. An ID may only be applied to a single element. Pseudo-classes are used in CSS selectors to permit formatting based on information that is not contained in the document tree. One example of a widely used pseudo-class is :hover, which identifies content 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 #elementid:hover. A pseudo-class classifies document elements, such as :link or :visited, whereas a pseudo-element makes a selection that may consist of partial elements, such as ::first-line or ::first-letter.[6] Selectors may be combined in many ways to achieve great specificity and flexibility.[7] Multiple selectors may be joined in a spaced list to specify elements by location, element type, id, class, or any combination thereof. The order of the selectors is important. For example, div .myClass {color: red;} applies to all elements of class myClass that are inside div elements, whereas .myClass div {color: red;} applies to all div elements that are inside elements of class myClass. This is not to be confused with concatenated identifiers such as div.myClass {color: red;} which applies to div elements of class myClass. The following table provides a summary of selector syntax indicating usage and the version of CSS that introduced it.[8]