Some time ago, some kind soul posted a Firefox bookmarklet that's very
handy for looking up a TEI element -- you either select the element name
on a web page and click on it, or click on it without selecting anything
and get prompted to type or paste the element name in, and then it takes
you straight to the Guidelines spec for the element.
It recently stopped working after an upgrade to Firefox, and I've
figured out how to get it working again, so in case anyone else is using
it, this is functional with current versions of Firefox: