As for why the trailing dot is disallowed, see <http://saynt2day.blogspot.com/2013/03/danger-of-trailing-dot....

The goal was to come up with a good regular expression to validate URLs as user input, and not to match any URL that browsers can handle (as per the URL Standard).

a.b--c.de is supposed to fail because `--` can only occur in Punycoded domain name labels, and those can only start with `xn--` (not `b--`).