I blogged earlier about ability of VS 2008 and Visual Web Developer 2008 to display HTML and CSS validation errors as warnings. Unfortunately, there is a small bug in the setup so although dialog displays that default setting is 'warnings', in fact, CSS issues are still flagged as errors. To fix this please do the following:
1. Go to Tools | Options | Text Editor | HTML | Validation and set HTML/CSS to errors (uncheck the option). 2. Click OK. 3. Repeat #1 but now set the option to warnings. Click OK and you should be set.