[css-color-4] Clarify serialization of rgb()/rgba() functions that have `none` components

[weinig]

CSS Color 4 indicates that none components are only allowed in the space separated syntax for rgb() / rgba() functions, but since rgb() / rgba() functions are serialized in the comma separated style, this leads to a weird case where the serialized form is not reparseable if any of the components are none. For example:

rgb(255 0 none)

should currently serialize as:

rgb(255, 0, none)

which is an invalid value.

Seems like either none should be allowed in the comma separated legacy form or serialization should be updated to specify a different serialization if none is used.

[svgeesus]

Good catch!

Because the comma-separated form is preferred mainly due to greatest backward compatibility, and because none is new, it would seem to make sense to serialize in the non-legacy form in that case.

On the other hand, allowing none everywhere is a bit of a heavy hammer and I would also be fine with just saying that none serializes as 0 here because I can't really see a use case for preserving none in RGB values. And in HSL or HWB it will serialize as rgb() in any case, with conversion to 0 for the hue if that was none.

[tabatkins]

Yeah, just serializing as 0 in rgb() seems reasonable to me, too.

[svgeesus]

Does this apply to resolving values, or just to serialization? (I assume the latter but just wanted to check first)

[weinig]

I hope just serialization. Doesn't seem like there is a compelling reason here to change value resolution.