Delete extra spaces.
Permissive HTML atttribute names, restrictive tag names.

HTML5 allows attribute names to consist of any character except
space. However, to permit this, we must check that tag names start
with a letter, followed by zero or more letters or spaces. Otherwise
we'd false match things like "1 < 2 and 2 > 1".
Use `call-with-limits` to simplify random testing timeout.