Visual Studio validates XHTML 1.0 Transitional by default. There are custom solutions for validating HTML5. But if you are writing a site that uses any part of the Facebook API or the meta property tags then your HTML is probably invalid. Most people don't know to change their DOCTYPE to this:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML+RDFa 1.0//EN" "http://www.w3.org/MarkUp/DTD/xhtml-rdfa-1.dtd">
What I'd like to know is, does anyone have a way to get Visual Studio 2010 to recognize RDFa like someone hacked it for HTML5?