In v3.6 another URL Protocol has been added (steam). This makes it possible to force users to upgrade their client for Steam-support. The new protocol goes something like this:
<a href="steam://[ip]:[port]> Click and FRAG! </a>
Of course, both protocols (halflife:// & steam://) will work just the same, only difference being that steam:// forces the user to upgrade from previous versions.