HTMLForm vform styling

Style <select> like other input fields (full width), and fix the issue
where validation errors partially obfuscate the fields by setting display:
block. Refactor code for styling error boxes and improve documentation.

Bug: 63644
Change-Id: I00a35c932a7e0b91b7b01fc327c0c1b9bae66c78
Merge "HTMLForm vform styling"