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."
Click and drag the link below to your bookmark toolbar to install - or right click "Bookmark This Link".
debugCSSGet the source code on GitHub : yahoo/debugCSS