When and how to validate HTML using contenteditable?

I'm exploring the idea of using contenteditable on a website at visitor comments and at a message board.

As I only wish to accept a white-list of HTML tags and attributes the question of wh开发者_StackOverflowen and how should I validate the entered HTML content comes up.

How would you do it? What cross-browser solutions would you choose to use? What events could be useful?

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HTML Purifier is getting momentum these days.