The difference is that parsing as the `color` property's value also
allows the CSS-wide keywords, which we don't want here.
The added test cases make sure that those keywords are *not* valid:
- `color` should inherit its parent value of `orange`
- `background-color` doesn't inherit, so should be its initial value of
`transparent`
`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.
