A term extended from print publishing to online media, a style sheet is a definition of a document's appearance in terms of such elements as:
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- The default typeface, size, and color for headings and body text
- How front matter (preface, figure list, title page, and so forth) should look
- How all or individual sections should be laid out in terms of space (for example, two newspaper columns, one column with headings having hanging heads, and so forth).
- Line spacing, margin widths on all sides, spacing between headings, and so forth
- How many heading levels should be included in any automatically generated Table of Contents
- Any boilerplate content that is to be included on certain pages (for example, copyright statements)
Typically, a style sheet is specified at the beginning of an electronic document, either by embedding it or linking to it. This style sheet applies to the entire document. As necessary, specific elements of the overall style sheet can be overridden by special coding that applies to a given section of the document.
For Web pages, a style sheet performs a similar function, allowing the designer to ensure an underlying consistency across a site's pages. The style elements can be specified once for the entire document by either imbedding the style rules in the document heading or cross-referring (linking to or importing) a separate style sheet. A browser may allow the user to override some or all of the style sheet attributes.
A cascading style sheet is a style sheet that anticipates that other style sheets will either fill in or override the overall style sheet. This provides the designer the advantage of being able to rely on the basic style sheet when desired and overriding it when desired. The filling in or overriding can occur on a succession of "cascading" levels of style sheets. For example, one style sheet could be created and linked to from every Web page of a Web site as the overall style sheet. For any portion of a page that included a certain kind of content such as a catalog of products, another style sheet that amends the basic style sheet could be linked to. And within the span of that style sheet, yet another style sheet could be specified as applying to a particular type of product display.
When creating Web pages, the use of style sheets is now recommended by the World Wide Web Consortium. The latest version of the Hypertext Markup Language, HTML 4.0, while continuing to support older tags, indicates which ones should be replaced by the use of style sheet specifications. The Web's Cascading Style Sheets, level 1 (CSSL1) is a recommendation for cascading style sheets that has been developed by a working group of the World Wide Web Consortium (W3C).