Because it uses whitelists and a comprehensive knowledge of the HTML specification, it is bullet-proof against XSS, fixes malformed input rather than reject it, and is open and extensible. The 2.0.0 release lets users customize HTML Purifier's tag sets easier than ever before.

You can check out the official release announcement for of the updates in this release (including a comparison to other similar libraries). There's also a demo they've posted so you can give it a try.