The reason I was looking for this:
What I'm trying to do is make keys like the arrows and enter navigate through a table of input elements. Tab works now, but enter and arrows do not by default it seems. I've got the key handling part set up but now I need to figure out how to move the focus over in the event handling functions.
document.activeElement, it is supported in all major browsers.
Previously, if you were trying to find out what form field has focus, you could not. To emulate detection within older browsers, add a "focus" event handler to all fields and record the last-focused field in a variable. Add a "blur" handler to clear the variable upon a blur event for the last-focused field.