Anith Gopal

Menu:  Tools -> Options -> Text Editor -> HTML -> Validation Versions:  2008,2010 Published:  12/9/2010 Code:  vstipEdit0084   Normally HTML and CSS syntax problems will show up as warnings (green squiggles):   These also show up as warnings in the Error List window:     This means you can build and run the application if you choose to ignore the warnings.  You can choose to have these show up as errors instead if you want by going to Tools -> Options -> Text Editor -> HTML -> Validation and deselecting “as warnings”:     Now the green squiggles will be red:   And the previous warnings will show up as errors in the Error List window:

See more here:
Display HTML/CSS Warnings as Errors