From the blog post: http://neilkilbride.blogspot.com/2007/10/visual-studio-error-list-html-errors.html.
Nothing annoys me more on a Friday afternoon than the Visual Studio Error List rearring its ugly head and popping up with errors (often irrelevant ones) about the HTML markup.
So after playing about with settings and poking around the interweb I found you can either just close all the HTML (or aspx) files you are working on (as it only critises open document's markup) or you can tell Visual Studio to ignore the errors by unchecking:
Tools > Options > Text Editor > HTML > Validation > Show errors
Please, no comments about using the correct validation target, actually fixing the errors or conforming to how Visual Studio wants me to do everything - it's my party and I'll do what I want :P
Regards,
Serge van den Oever
http://weblogs.asp.net/soever