Answer:
Technically, there is no such thing as an HTML program, since HTML is a markup language. However, a page of HTML content may be included within another page in several ways.
Using a server-side processing language such as PHP, two source files may be combined:
<?php include "page2.html"; ?>
Using server-side includes on a supported server, files may also be combined: <!--#include file="page2.html" -->
There are also client-side means. A page may be loaded inside an <iframe> tag, for example:
<iframe src="page2.html" />
Finally, Javascript may be applied on the client-side to include pages. This is a little more complex:
<script type="text/javascript" src="page2.js" />
Content of page2.js:
document.write("HTML for page2.html");