CSS, HTML & XML Test and Repair Tool.
Tart can validate the most common types of web resources: (x)html, css, rss, atom and sitemaps, so you can focus on testing and fixing problems instead of finding the right validator. Plus Tart runs on your workstation, so validating local and private resources is a snap.
Tart allows you to categorize and filter diagnostics, so you can utilize WAI-ARIA, webkit, mozilla, or other extensions without having to sift through the diagnostics reported to find the real problems. Simply create a category, and drag diagnostics to it. Then you can see the diagnostics that you want when you want.
Unlike other validators that just tell you where problems are, Tart has an integrated editor, so you can quickly get to the source of the problem, fix it, and retest, all in a single window.