Wow, TIL it's a user-preference option (Preferences->Browsing->"Press Tab to highlight each item on a webpage"). Don't really have a good source to link to for this, but in case someone else comes across this thread, there's the solution at this point in time.
There is a Webkit issue out there, which aims to enable tab navigation for more form elements without having to check the user-preference option. You can find that here.
