Just to add a little to the SSI. Generally, web servers will have already enable SSI for .shtml and to enable SSI for other extension (i.e .htm .html), you will need access to .htaccess in your 'public_html' folder, enter the following code in it :-

AddType text/html .htm
AddHandler server-parsed .htm
Options Indexes FollowSymLinks Includes

Above will enable SSI for files with .htm extension.

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22 steps on how to create a good website
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