Tart

Tart is a tool to validate the majority of common web resources: HTML, CSS, RSS, Atom, and sitemaps. This way you can focus on solving problems instead of wasting time finding the correct validator.

The program lets you categorize and filter diagnostics, making it much easier to find problems. Just create a category and drag the diagnostics so you always have them perfectly organized.

Unlike similar applications that only tell you where the problem is, Tart includes an editor so you can quickly get to the source of the problem and fix it – all in one single window.