I have some old web content that works using Server Side Includes and I needed to get it functional on a new web server. I found this article How do I enable server-side includes (SSIs) on my site? which had everything I needed to know.

First I needed to add the content type and output filter to the Apache configuration:

  AddType text/html .shtml
  AddOutputFilter INCLUDES .shtml

Then I needed to add the IncludesNoExec option:

  Options MultiViews Indexes SymLinksIfOwnerMatch IncludesNoExec

Then I needed to specify the DirectoryIndex:

  DirectoryIndex index.shtml index.html

And that was it!