HTML Purifier is an HTML filtering solution that uses a unique combination of robust whitelists and aggressive parsing to ensure that not only are XSS attacks thwarted, but the resulting HTML is standards compliant.
HTML Purifier is oriented towards richly formatted documents from untrusted sources that require CSS and a full tag-set. This library can be configured to accept a more restrictive set of tags, but it won't be as efficient as more bare-bones parsers. It will, however, do the job right, which may be more important.