[css-backgrounds-3] Computed value of background-size includes missing autos

[csnardi]

https://drafts.csswg.org/css-backgrounds-3/#the-background-size says the computed value of background-size is:

as specified, but with lengths made absolute and omitted auto keywords filled in

https://wpt.fyi/css/css-backgrounds/background-size-001.html shows that only Firefox outputs the computed value with missing autos. It seems this was added in bfd84cf by @fantasai, but I don't see any reasoning there. Perhaps this should be changed to remove the portion about omitted auto keywords, as it would also conform to the shortest serialization principle outlaid in #1564.

[BorisChiou]

I'm working on updating the serialization of background-size on Firefox. I think the serialization of 12px auto as 12px auto (instead of 12px) is also pretty weird to me, and it makes the implementation more complicated. I'm happy to update the wpt to satisfy the shortest serialization principle in the same patch. (So all browsers can pass all the tests in background-size-001.html)

cc @emilio @dholbert

[BorisChiou]

Actually, what I thought is:
For example,
(input => serialzation of the computed value)

1. auto => auto
2. auto auto => auto
3. 12px => 12px
4. 12px auto => 12px

In other words, we omit the 2nd value if it is auto.
Both Chrome and Safari follow this rule, and it makes sense to me.

[emilio]

Agreed, that seems more correct than what FF is doing right now. Will add it to the agenda to get a resolution from the WG, but I assume this is quite uncontroversial if it matches both WebKit and Blink.