The premise of revenge.css is simple: A CSS bookmarklet that uses selectors to find bad markup, displaying ugly pink error messages in comic sans serif wherever you write bad HTML. If you activate the bookmarklet and the page gets lots of pink blotches, the author has included at least one of the following:

• Misplaced <div>s
• Deprecated elements
• Malformed hyperlinks
• Inaccessible forms
• Empty elements
• Inaccessible images
• Missing ARIA landmarks
• Badly authored sectioning elements
• Erroneous lists
• Obsolete attributes

GitHub won't let me use javascript in my README. Pretty sensible. Go to the new hot pink, skull-festooned demo page.