It wont work on HTML pages. Its supported by ASP.
This is an ASP directive.
in ASP.net we have supposingly header.ascx then we first
<%@ Register TagPrefix="Abc" TagName="Header" Src="includes/header.ascx" %>
then in the body where we want to place the include file
<Abc:Header ID="myHeader" Runat="Server"/>
These work only on server side scripts like ASP, ASP.net, PHP, JSP etc. These are compiled before being displayed by the server. Like the forum is n PHP and if view the source you will see plain HTML. The PHP was compiled and all such directives converted to routine HTML.