Thanks, Jens!

I encourage feedback. The point I made in the first article of this series is that there is no feedback for a disabled button for those using assistive technology or those using a keyboard.

As for the Proximity Feedback you linked, it's pretty spiffy - but only if you are a sighted mouse user. It provides nothing for keyboard-only users. In fact, the Register button doesn't have a focus indicator, which doesn't even meet WCAG.