Server Side Includes
Server Side Includes (SSIs) are allowed, but require the calling page to have the extension .shtml instead of .html. Typical syntax for includes would look like:

<!--#include virtual="includes/includedpage.html" -->.

Here the section "includes/includedpage.html" identifies the path to the page to be included. If it is in the same directory, just use the file name, otherwise provide a path to the file (eg. the "includes" directory listed above.

Note that the <!--#exec ...--> command is not allowed for security reasons. Nonetheless, you may use CGI scripts as an alternative if you need to generate dynamically executable pages.