It is practically unlimited as far as extensions. With UNIX, you can tell it to parse an HTML page like a PHP page etc. You can even do it so as to drop the extension

I usually use ASP so that I can use includes. Since we host on a Windows server, I know I can use SHTML, but since it is the same DLL that parses the SHTML files and the same load / CPU usage, we just use ASP. I tried to create one awhile back in HTML and it just did not work out. When I added a new web page, I had to go thru and add that on all the pages.