[css-values-4][cssom] Serialization of <position>

[nt1m]

https://drafts.csswg.org/css-values-4/#position-serialization says:

If only one component is specified:
The implied center keyword is added, and a 2-component value is serialized.

but https://drafts.csswg.org/cssom/#serializing-css-values says:

If component values can be omitted or replaced with a shorter representation without changing the meaning of the value, omit/replace them.

Which one is right? Personally I have a slight preference of omitting redundant values, unless there's a specific compat reason not to (which is mandated by the CSSOM spec), just for the sake of consistency with serialization of other types.

[nt1m]

cc @emilio @fantasai @tabatkins @danielsakhapov @Loirooriol

[Loirooriol]

https://wiki.csswg.org/ideas/mistakes

background-size with one value should duplicate its value, not default the second one to auto. Ditto translate().

I guess that a reasonable expectation would be that background-position: 0px behaves as background-position: 0px 0px, so serializing as background-position: 0px center can avoid misunderstandings.

See whatwg/compat#28 and #7802.

[nt1m]

The citations you linked is for background-size (which is 2 sizes), not background-position, not sure there's as much confusion for background-position

[Loirooriol]

Oh I mixed them up in my mind, sorry. But it's still the case that background-position: 0px means 0px center instead of repeating. So there may still be confusion? I don't know.

[nt1m]

Yes, background-position: 0px means 0px center, not 0px 0px

[cdoublev]

fantasai: a more important reason is the transform-origin syntax, which becomes ambiguous if you only have one value
fantasai: You wouldn’t be able to consider position as a single value without considering whether or not you have a Z component. That creates a serialization of position cannot be self-contained
fantasai: The proposal: always serializes at least two values

#2274 (comment)

[tabatkins]

Ah, I forgot about that when I approved the test change that reduced some position serializations. We should capture that in the spec, since it's important.