The only difference between regular HTML and SHTML is the extra letter in the extension (.shtml) and this code.

<!--#include file="addedfile.txt" -->

The S stands for 'Server Side Include' or SSI. When an SHTML webpage is sent to the web browser window, it gets assembled on the server and then sent to be viewed. The normal HTML tags all still work the same, the SHTML simply lets you INCLUDE other pieces into the HTML page.