My solution to the Tip Calculator App

The HTML and CSS for this was done in two days, but the JS tied me down. Also, I was confused why there was no 'submit' button for this since this was a form. Anyway, calculator apps also behave like this, so...

Recognized a previous challenge can be included here and that was the Interactive Rating Component. It would be used where the user would select the tip in percent, but what about 'custom'? I thought about it and ended up using six radiobuttons with the last tied to an input field as 'custom'. I utilized event delegation to avoid assigning too many event listeners. A lot of tiny little things had to be tracked in order for keyboard navigation to behave in a standard manner.

Have a look see. If anyone can look at my JS and offer general tips for improvement I'd be grateful.