Adress more #1988 (Acid 2) review comments.
Computed value of 'height: <percentage>' is 'auto' in some cases.

See http://dev.w3.org/csswg/css2/visudet.html#propdef-height

This is necessary but not sufficient for #2029.

A bug fix in the previous commit "broke" Acid 2 because that bug was hiding
this one. This makes Acid 2 pass again.