First, shut down VS2010
Assuming you're validating against html5 - the validation option is in
Text Editor-->HTML-->Validation
Set it to HTML5 validation. Then verify the following file exists:
< your install directory >\Microsoft Visual Studio
10.0\Common7\Packages\schemas\html\html_5.xsd
Copy the file elsewhere to be safe then edit the original file using a text editor. Near the top of the file find the string:
vs:cssschema="CSS 2.1"
and change it to:
vs:cssschema="CSS 3.0"
if it isn't already. Then save the file and restart VS2010. That should take care of making CSS 3.0 validation sticky against html5 projects