You can now automatically vary the HTML + client-script intellisense you receive in the editor based on either a standards based schema (html 4.01, xhtml transitional, xhtml strict, etc) or for a specific browser device (IE 6.0, etc).

There is now a device drop-down list in the toolbar that allows you to easily toggle between the different schemas:

Figure 4

This will then update all HTML/client intellisense settings (so you only see those elements/attribute/script object model values legal for the particular schema target you have picked):

Figure 5

Note that the default schema validation value for new web projects is now XHTML Transitional.  You can also easily add your own schemas to VS if you want to modify any of the built-in ones or publish your own.