The height and depth attributes are parsed individually and then
combined into a `calc()` expression. Bare zero is valid as a standalone
CSS length but inside `calc()` it is typed as a number, so
`calc(0 + 0)` fails to parse as a length. We now check the parsed value
is valid to avoid a crash.
`HTML::parse_dimension_value()` doesn't parse units except for `%` for
percentages; it just ignores them and treats it as a number of pixels.
Now that we can parse `<length>` and pals directly, do that instead,
which makes non-px units work.
