What is HTML Purifier?
HTML Purifier is a standards-compliant HTML filter library written in PHP. HTML Purifier will not only remove all malicious code (better known as XSS) with a thoroughly audited, secure yet permissive whitelist, it will also make sure your documents are standards compliant, something only achievable with a comprehensive knowledge of W3C's specifications.
Muy interesante para implementarlo en los puntos de entrada de datos, por parte de los usuarios, de un sitio web.