Fix strange mw.html errors with numeric arguments

Some functions in mw.html accept numbers as arguments, but later fail when
constructing the string. This disallows numbers in attribute names, since
they aren't valid anyway, and fixes the remainder of the cases to properly
build the string.

Bug: 67201
Change-Id: Ie7bcbb9d8df580dd8793681f78a8b0719d8a287a