I didn't say there are cross-site scripting issues known right now, but I think in accepting information from 'untrusted' sources, one can't be paranoid enough. My point is that we don't know how harmful any given tag is because we can't be sure there are no bugs in the browser implementations (<insert rant about non open-sourced software here>). And even open-sourced browsers might contain bugs. Given the fact that in the past even simple things like <img>-tags have been exploitable in certain browsers, makes me think one can not be careful enough. Be prepared for the unexpected.

And I also am not totally against adding (some of) these tags (after all I don't have to do the coding), I just wanted these thoughts taken into consideration.