From what I can see in WebKit source (the only one I looked at), basically it *must* be a HTMLElement in order to be able to contain interpreted JS. In effect, if it isn't a HTMLElement (could check after parse, or after it is imported) it probably can't contain JS.

You don't have to guess on this either... the test harness branch I created should allow you to easily check how these script elements are parsed. (My own investigation showed Chrome, FF + IE interpreted the xhtml namespace equivalent to document.createElement(...)).