I blogged about some of my favorite html source editor features in Visual Web Developer and VS 2005 a few months ago. In case you missed it, you can read that post here.
One of the features I mentioned was the ability to get per-browser/device/standard intellisense and validation. The ensures that the editor only shows you those elements, attributes and client-side javascript OM that is supported by the particular browser or standard you are targetting.
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,
Scott
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.