[css-backgrounds-4] Computed Value serialization of background-position and its longhands

[javifernandez]

In L4, the background-position is defined as a shorthand of background-position-x and background-position-y, but its Computed Value definition remains the same than in L3:

https://drafts.csswg.org/css-backgrounds-4/#the-background-position

list, each item a pair of offsets (horizontal and vertical) from the top left origin each given as a computed value

So it seems the computed value it's serialized without the origin keywords. Even more, the <bg-position> is defined in the as <position>|<position-three>' and there is a NOTE in the CSS Value 4 spec regarding the <position> computed value that explicitly states to remove the origin keywords in the serialization.

Hewever, the longhands defined in the L4 state that their computed value must be serialized as the offset plus the origin keyword:

https://drafts.csswg.org/css-backgrounds-4/#background-position-longhands

A list, each item consisting of: an offset given as a computed value, plus an origin keyword

I think the reason why we need the origin in the longhands it's because the logical values x-start and x-end added to the syntax in L4, but I'm not sure I understand why we don't need the origin keywords in the shorthand too.

I just wanted to be sure this is the intended behavior because there are a few tests already checking for the absence of the origin keywords in the background-positionproperty. It seems that only chrome fails in the case of the <position-three syntax. It might be due to a bug in Chrome (although not the one registered in wpt.fyi), but I wanted to be sure the origin keywords are not going to be required in the shorthand.

Finally, there are also tests for the longhands, where it's expected that the computed value omit the keyword (eg left -20px expects 20px). So far it fails in Chrome because the longhands x/y lacks support for the 2 values syntax (bug 40468636), which I'm working on right now, but again I'd need some clarification about the origin keyword in the computed value serialization.