Visual Studio 2005 and Visual Web Developer Express 2005 display markup and CSS validation issues such as unuspported tags and attributes as errors. Some people like it since validation errors encorage standard compliance. Others didn't want validation to pollute Error List window and would rather see markup and CSS issues as warnings. We listened and added the feature after Visual Studio 2008 Beta 2. In the RTM you will find two extra options:

When option is checked, HTML markup and CSS validation errors become warnings and warnings (such as deprecated attribute warning) become messages. Since VS 2008 now has ability to syntax check JScript code, we also added an option to display JScript errors as warnings as well.