Previously, presentational hints bypassed the regular cascade pipeline
and wrote directly into `CascadedProperties` under
`CascadeOrigin::Author`. That meant `var()` substitution and the
invalid-at-computed-value-time fallback had to be duplicated in a
separate per-element pass, which in practice missed the IACVT step and
could leave a `GuaranteedInvalidStyleValue` in the cascaded
properties. This caused a crash in downstream code that assumed the
value had been resolved.
This introduces an `AuthorPresentationalHint` cascade origin and feeds
them through the cascade as normal declarations. This means that
`var()` resolution now happens in only one place.
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.
