[cssom-1] Spec disagrees with WPT on resolution and time serialization

[AtkinsSJ]

The spec says to always serialize a <resolution> as dppx:

<resolution>
The resolution in dots per CSS pixel serialized as per <number> followed by the literal string "dppx".

However, actually doing this breaks a few subtests of https://wpt.live/css/mediaqueries/match-media-parsing.html , because the media queries are expected to serialize using their specified units.

Technically, media query serialization doesn't use that value-serialization part of the spec, but it's mostly missing so I think it's reasonable to serialize media-feature values like regular property values.

The definition for <time> also specifies to always serialize in s units. Other dimensions serialize using whatever their specified unit is. Is this distinction important? I can't think of a particular reason why these would serialize differently. I'd actually propose combining all the dimension branches in https://drafts.csswg.org/cssom/#serialize-a-css-value together, using the text for <length>:

The <number> component serialized as per <number> followed by the unit in its canonical form as defined in its respective specification.

EDIT: <time> is a more compelling case: Both css/css-transitions/parsing/transition-duration-valid.html and css/css-transitions/parsing/transition-delay-valid.html expect it to be serialized using its specified units.