On 23 Oct 2006, at 7:13 PM, Konstantin Käfer wrote:

The reason why filter.module removes style tags is simple: some dumb browsers allow JavaScript inside stylesheets, for example "font-size:expression(prompt('Enter a font name:', 'Arial'));". Using that you could execute potentially harmful JavaScript code that allows for XSS.

and using the full html 'filter' lets them do that without having to jump through hoops even.