<blockquote>though .css etc. can generate exceptions in such an environment,</blockquote>
It will not, <code>jsdom</code> has <code>cssstyle</code> package dependence, so it will work just fine, and it still a weird case when jQuery would be used without <code>node-jquery</code> <a href="https://github.com/coolaj86/node-jquery/blob/master/src/wrapper.js">wrapper</a>.

<blockquote> I think I'm back to supporting this if ( !a ) early abort so we don't fail on div.style.backgroundClip</blockquote>
Why? So jQuery would not throw error in FF and Opera if it used in xml document? That's a weird case too, jQuery would still be broken, if it would be silent about it – how is that better?