The following is from the: Techencyclopedia Web site / techweb.com
(Cascading Style Sheet) A style sheet format for HTML documents endorsed by the World Wide Web Consortium. CSS1 (Version 1.0) provides hundreds of layout settings that can be applied to all the subsequent HTML pages that are downloaded. CSS2 (Version 2.0) adds support for XML, oral presentations for the visually impaired, downloadable fonts and other enhancements
Style Sheet:
A master page layout used in document creation systems such as word processing, desktop publishing and the Web. The style sheet is a file that is used to store margins, tabs, fonts, headers, footers and other layout settings for a particular category of document. When a style sheet is selected, its format settings are applied to all the documents created under it, saving the page designer or programmer from redefining the same settings over and over again for each page.
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CSS's are used to format Web pages without having to apply the formatting to every single Web page element individually. Rather than place a format attribute into each HTML tag, a Web author can define what attributes will be applied to any elements assigned to an HTML tag by predefining them in a cascading style sheet. CSS's can be executed from the "header" of an HTML page. However each page needing a style sheet would have to have it's own CSS in order for this method to work.
SSI (Server Side Includes) allow Web pages to pull CSS's from the Server. Therefore in theory a Web author can change the attributes of one style sheet and dynamically effect the appearance of an entire Web site. Nevertheless in practical application Web authors generally have multiple style sheets that apply to different types of pages within a site or different themes within a given site.