If you have tried adding in SSI include codes into your HTML files, but find that it doesn’t work, the problem may be that you might not have enabled SSI yet.
The easier way to do so is to edit your .htaccess file to have these lines of code:
AddHandler server-parsed .html
Options Indexes FollowSymLinks Includes
By setting this, you are telling your server to parse all HTML files. There’s a drawback though — ALL HTML files are parsed — even if they don’t have SSI or include code in them. So, there’s a bit of an overhead. Otherwise, this is fine.