Red squiggles correspond to validation errors such as a missing closing tag. Green squiggles correspond to validation warnings such as the use of deprecated tags.
See how easy that was? Seriously, Stephen Walther, author and developer extraordinaire, has a new article at MSDN entitled, "Building ASP.NET 2.0 Web Sites Using Web Standards."
I say it should be required reading for EVERY developer who is using or getting ready to use ASP.NET 2.0 (including a few book authors I can think about).
It's 53 pp printed out, and very well put together.
On a side note, I notice this morning that Release Candidate RC0 is dropped at MSDN Subscriber Downloads in advance of PDC 2005. As one would expect, the Subscriber login is overloaded and doesn't work.
And a further note on TDD and test coverage from a previous UnBlog post about test coverage, MS Press's new book by Newkirk on TDD is out: