>I hardly think they will add any more tags. Only a few tags have been added over the last few years, since most changes now are done via CSS. MSFT has stated that default for the next browsers will be transitional HTML, but may change to XHTML sooner than we think. I guess Opera, Chrome, Firefox and the rest will follow the same pattern.

Well, because of some issues in the past, I have to use the secure route. I cannot maintain a list of all HTML tags, XML tags and so on. I can only allow what should be allowed. That is the secure route to use.