What is cool about the html source engine is that it is extensible, and you can add your own schemas to the built-in list that ships with the product (all you need to-do is author your XML schema file). Mikhail has just posted a blog entry that walksthrough the steps to build a new one. You can read all about it here.
Hope this helps,
P.S. If you don't like having HTML validation on (but do want HTML intellisense), you can check out this blog post I did that talks about how to turn it off.