[css-backgrounds] What is the serialization "background-position: top right 10px"?

[smfr]

https://www.w3.org/TR/css-backgrounds-3/#background-position doesn't say how the edge-relative background position values serialize (like background-position: top right 10px).

This is tested by css/css-backgrounds/parsing/background-position-x-computed.html and css/css-backgrounds/parsing/background-position-y-computed.html which in Gecko test that right -10px serializes as calc(100% + 10px) and bottom -10px serializes as calc(100% + 10px) but the spec does not describe this.

[emilio]

FWIW those tests were added in web-platform-tests/wpt#16867. I agree that this is underdefined. The calc() serialization in Gecko just comes from this line of code where we remove the keywords when going from specified -> computed value.

[fantasai]

The spec defines the computed value as “list, each item a pair of offsets (horizontal and vertical) from the top left origin each given as a computed <length-percentage> value”.

The only way to serialize that would be as a pair of length, percentage or calc() values, possibly with the values 0, 0%, 50%, and 100% converted to equivalent keywords (and the question of whether to do so being independent of whether one, two, three, or four components were specified).

[css-meeting-bot]

The CSS Working Group just discussed css-background-3, and agreed to the following:

• RESOLVED: Add a note

The full IRC log of that discussion

<fantasai> Topic: css-background-3
<fantasai> github: https://github.com//issues/6586
<drott> chris: > @drott I see the PR on your fork repo but not the one on the csswg repo I'll turn it into a PR visible on the CSSWG repo.
<fantasai> smfr: Spec allows specifying a corner and an offset from that corner
<fantasai> smfr: spec doesn't specify
<fantasai> smfr: WebKit serializes as inputted
<fantasai> smfr: Gecko serializes calc()
<drott> fantasai: we already did discuss this - that's the way the serialized value is the way it is.
<drott> fantasai: you could possibly map the values of varying percentages to keywords.
<fantasai> s/serialized/computed/
<drott> fantasai: it's said that it's a length/percentag value per axis. there's no way to distinguish if somebody specified calc() of a value
<drott> fantasai: we had discussed this before: main concern would be: in the future we would have logical keywords
<drott> fantasai: with that, the computed value for background-position would become more complicated
<drott> fantasai: i don't have a strong opinion on it because of what we plan for in the future.
<fantasai> fantasai: spec is clear, in the "computed value" line
<emilio> I agree this falls from the "Computed Value" definition
<fantasai> TabAtkins: Since it confused some people, maybe a note to highlight that this information is lost would be helpful
<fantasai> smfr: I just wanted to follow link to where it's clear...
<fantasai> TabAtkins: as a combination of lengths and percentages, you'd have to use calc()
<fantasai> dbaron[m]: other principle is use most backwards-compatible syntax, and at least in Gecko calc() predated the background-position extended syntax
<fantasai> astearns: Suggest closing issue by adding a note to css-backgrounds-3 and maybe also values and units
<fantasai> smfr: wfm
<fantasai> RESOLVED: Add a note
<fantasai> astearns: no normative change, but we will explain why

[fantasai]

This was edited in 0742ad6