Enabling SSI On Non-.shtml Web Pages
SSI can be enabled on .html or .htm web pages without renaming the entire site with .shtml file name extensions.
Inserting these two lines into the .htaccess file should do the trick:
AddType text/html .shtml .html AddHandler server-parsed .shtml .html
The above will cause the server to parse every .shtml and .html web page for SSI tags before it is sent to the browser.
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Will Bontrager