DebugCSS — (X)HTML debugging tool built with CSS
debugCSS is meant to be loaded on an existing page to highlight potentially broken, malformed or legacy (X)HTML.
Not all "errors" are created equally, so they are color coded to highlight severity. Green is "probably not a big problem", yellow is "worth looking at" and red is "you really should fix this."
Read more in http://yahoo.github.io/debugCSS/