Another great project from OWASP: AntiSamy. It is a collection of APIs for safely allowing users to supply their own HTML and CSS without exposing the site to XSS vulnerabilities.
Fig.1 AntiSamy DEMO.
The methodology of AntiSamy is unique in that it is built on a positive security model in both the format of the HTML document and the content within the document. It's also unique in that it attempts to help the user tune their input to pass validation in a cooperative spirit, rather than treating users as potential attackers which is how all contemporary security mechanisms work
Philosophically, AntiSamy is a departure from contemporary security mechanisms. Generally, the security mechanism and user have a communication that is virtually one way, for good reason. Letting the potential attacker know details about the validation is considered unwise as it allows the attacker to "learn" and "recon" the mechanism for weaknesses. These types of information leaks can also hurt in ways you don't expect. A login mechanism that tells the user, "Username invalid" leaks the fact that a user by that name does not exist. A user could use a dictionary or phone book or both to remotely come up with a list of valid usernames. Using this information, an attacker could launch a brute force attack or massive account lock denial-of-service. We get that.