The following is from the: Techencyclopedia Web site / techweb.com
Sever Side Inclulde:
An HTML command used to place up-to-date data or boilerplate into a Web page before sending it to the user. For example, it can be used to retrieve the current date and size of downloadable files that are constantly changing. It can be used to insert a boilerplate message where only the boilerplate needs to be changed to bring all the pages up-to-date. HTML pages that contain server-side includes often use the .shtml file extension.
The "Include" command inserts the contents of another document at the "tag" location. "Echo" inserts the contents of an environment variable. "Fsize" and "Flastmod" insert size and date of a file, and "Config" controls the format of the output. The "Exec" command executes a CGI script.
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SSI's also 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.