Currently, the contents of noscript, noframes, iframe...etc., ( tags that contain alertnate content ) are collected as string since the content should not be processed when script/frames are enabled. However, the collected content is never used, in the browser mode, and hence does not have to be in the content model. That is, the parser can dispose the alternate content in the browser mode ( but should retain it for editor ).

But if someone turns off Javascript, shouldn't we display the content of "noscript"? (I think there's also a bug asking for the ability to turn off frames, but I'm not sure.)

Yes if script is turned off then we treat the alternate content as regular content and will become a part of the content model.

Does this seem reasonable from a compat point of view? Are there sites that depend on this being in the DOM?

It would always be in the content model for XHTML... I'd rather leave it in for HTML too, the fewer differences between the two the better.

Actually, for XHTML we insert 'real' DOM, i.e. elements etc so we are already not doing the same thing between the two. I.e. for the following markup: <noscript> <span>error</span> </noscript> In HTML create just a big textnode with .value = "<span>error</span>" In XHTML we create an actual <span> with a child-textnode with .value = "error". Actually, i'm not even sure that that is what we do for HTML, i have a recollection that we actually did drop that content. It could differ from alternate-content-element to alternate-content-element. However, I do think that we should move more towards what XHTML is doing, not towards what HTML is doing. Partly because of what hixie said; differences are good to avoid. But mostly because in many cases things don't work as they should in XHTML. Elements expect to be removed from the DOM when they are inside an alternate-content-element, if they are inserted they execute happily without checking if they actually should. The reason we don't find this is simply that people aren't using XHTML very much. But when they do they run into bugs like this (which makes them switch back to HTML many times i would guess).

This should be WONTFIX per HTML5.