[kevincox]

From reading a little bit of the code it sounds like Roundcube's sanitizer is much closer to a blacklist than a whitelist. Any attempt to sanitize HTML with a blacklist is doomed to failure. Even if you read the current HTML spec (including referenced specs like SVG) and do a perfect job there are additions over time that you will be vulnerable to.

Probably any unknown element attribute pair should be stripped by default. And that's still not considering different "namespaces" such as SVG and MathML that you need to be careful with.