[color-5] Serialization of percentages in color-mix()

[tiaanl]

In reference to this section:

csswg-drafts/css-color-5/Overview.bs

Line 2585 in 43b12e4

<h3 id="serial-color-mix">

The spec doesn't mention anything about serializing the incoming color-mix(), only the result. For color-mix() itself the tests suggest that the percentages on both sides should only be omitted if the author omitted them, otherwise they should be present.

@emilio mentioned that there is also the serializing-css-values section that suggests the percentages could be omitted if they are obvious.

So for example color-mix(in hsl, red 60%, blue 40%) should serialize to color-mix(in hsl, red 60%, blue). This contradicts what is currently in the tests.

As all other browsers (except firefox) is following the current wpt tests, I think we should maybe explicitly mention this in the spec?

[emilio]

I think we should fix the test instead FWIW. cc @svgeesus

[svgeesus]

The spec should clearly state what happens, even if it is just the logical application of a principle, and should have an example. I'm sure that lack of this clarity explains the divergent implementations on that point.

I agree that the tests do not follow that, and should be corrected.

[svgeesus]

So I just added this

The serialization of the specified value of a ''color-mix()'' function
is the string "color-mix(in ",
followed by the specified <<color-space>> in all-lowercase,
followed by ", ",
followed by the first specified color,
followed by a space,
followed by the normalized first percentage,
followed by ", ",
followed by the second specified color,
followed by ")".

Following the principle of shortest serialization,
and given that the normalized percentages are serialized,
the normalized second percentage is omitted.

but see a problem: if the normalized percentages add to less than 100%, how do we serialize the alpha multiplier (which cannot be expressed directly in the syntax, only indirectly.

Si I am thinking we need to retain the as-specified (un-normalized) percentages, if they are both given and if they add to less than 100%.

[svgeesus]

@tiaanl @emilio any thoughts? The only way I see to retain the alpha modifier is to retain the un-normalized percentages; we can do that just where they add to less than 100%, or do it always.

[emilio]

Here's what we do which I think is sensible. TLDR, you omit the right percentage if left + right add up to 100%, you omit both if left and right are 50% each, otherwise you omit none of them.

[svgeesus]

Okay, but is that the specified percentage or the normalized percentage (sounds like the specified one)?

So for color-mix(in oklab, yellow 70%, orange 70%) do you store as-is or do you normalize to 50% each and thus omit both?

[svgeesus]

So now we have

The serialization of the specified value of a ''color-mix()'' function
is the string "color-mix(in ",
followed by the specified <<color-space>> in all-lowercase,
followed by ", ",
followed by the first specified color,
followed by a space,
followed by the specified (un-normalized) first percentage (unless that is 50%),
followed by ", ",
followed by the second specified color,
followed by the specified (un-normalized) second percentage 
(unless either that is 50%, or the two specified percentages add to 100%),
followed by ")".