[css-color-5] Should relative `rgb()`/`rgba()` and relative `hsl()`/`hsla()` serialize exactly as declared?

[weinig]

In CSS Color 5, the serialization of relative colors is defined with the following:

The serialization of the declared value of a relative color function is a string identifying the color function in all-lowercase, followed by "(from ", followed by the serialization of the declared value of the origin color, followed by a single space, followed by a singly-space-separated list of the arguments to the color function, followed by ")".

Does this mean that when serializing rgb(from ...)/rgba(from ...) that the specific spelling as declared, rgb vs rgba should be preserved? Or is ok to canonicalize to one or the other?

Similarly, for hsl(from ...)/hsla(from ...) that the specific spelling as declared, hsl vs hsla should be preserved? Or is ok to canonicalize to one or the other?

[kbabbitt]

The spec goes on to state:

The serialization of the result of a relative color function depends on whether the keyword currentColor is the origin color. If so, the result is serialized as the declared value. This allows the correct value to be used on child elements whose color property has a different value. Otherwise, it is the resolved value, which is a <color>, as defined in CSS Color 4 §  15. Serializing <color> Values.

I understand this to mean:

1. If the origin color is currentColor, then serialize the spelling as declared. (We don't know the origin color and thus cannot canonicalize it.)
2. If the origin color is not currentColor, then resolve the relative color and serialize canonically as described in Color 4.

There are few tests with examples for how canonicalization happens in case 2: color-valid.html, color-valid-rgb.html, color-valid-hsl.html. Notably:

• hsla(...) canonicalizes to equivalent rgba(...), and hsl(...) canonicalizes to equivalent rgb(...)
• rgb(...) is used if the clamped alpha is exactly 1, otherwise rgba(...) is used

cc @svgeesus to confirm.

[weinig]

I filed a separate issue on the ambiguity here - #10305, as I don't think the current spec text is clear on what the rules are.

Regardless of that however, the question posed here remains, as you can just set currentColor as the origin:

That is, should:

rgb(from currentcolor r g b)

and

rgba(from currentcolor r g b)

serializations maintain the exact spelling of function name.

@kbabbitt, the tests you mention, notably, cover the absolute case, which has clear rules about which spelling to do, and those rules are not about what spelling was used at the declaration site, but rather about the resolved value of the alpha channel. As you might not know the value of the alpha channel in a relative color, the exact same rules can't be followed.

[annevk]

cc @LeaVerou

[cdoublev]

Fwiw, I would prefer to avoid omitting alpha >= 1 and canonicalizing rgba?() and hsla?() depending on whether it is omitted (edit: never mind, I have no preference), but I do not know the "historical reasons" why browsers need to resolve them when serializing them as components of declared values.

[weinig]

I don’t think the historical concerns apply to the relative form as there is no existing use to worry about breaking.

[LeaVerou]

Yup, I think it's fine to do whatever is reasonable in this case, since the historical concerns don’t apply.

[weinig]

My preference here would be to serialize:

rgb(from ...) and rgba(from ...) as rgb(from ...)
hsl(from ...) and hsla(from ...) as hsl(from ...)

This loses no useful information, and is minimal, which generally CSS serialization tries to achieve.

It's not a huge deal, implementation wise, to preserve the specific spelling for serialization, so whatever is decided here is fine.

[LeaVerou]

That makes sense. Agenda+ so we can resolve.