[css-color-5] Mixing `<number>` and `<percentage>` in non-legacy `<rgb()>`

[cdoublev]

The current value definition of <rgb()> is:

rgb( [<percentage> | none]{3} [ / [<alpha-value> | none] ]? ) |
rgb( [<number> | none]{3} [ / [<alpha-value> | none] ]?
rgb( [ from <color> ]? [ <number> | <percentage> | none]{3} [ / [<alpha-value> | none] ]? )

I think only the last line should remain.

Also discussed in #7338.

[svgeesus]

That one is odd, agreed, but deliberate, and an attempt at backwards compat.

The first two lines are CSS Color 4 formats (without from <color>) and require either three number or three percentage. The third line is Relative Color Syntax, and we allowed number and percentage to be freely mixed there (because people kept mixing them in examples).

However, given that these are all the non-legacy comma-less form, and given that none was a relatively late addition, I am not sure that this is really buying us much in terms of Web compat or browser compat.

@tabatkins @LeaVerou @argyleink @una thoughts?

[argyleink]

I could understand the first 2 being removed also, as the 3rd seems encompassing of the syntax options. Alternatively, I could see the first two being moved into an aside and provided as simpler, less feature rich and legacy, versions of the syntax for those looking for it?

[cdoublev]

My immediate concern is is the missing ) | at the end of the second line.

[LeaVerou]

I could understand the first 2 being removed also, as the 3rd seems encompassing of the syntax options. Alternatively, I could see the first two being moved into an aside and provided as simpler, less feature rich and legacy, versions of the syntax for those looking for it?

That would mean things like rgb(255 40% 100%) are valid. Do we want that?

[romainmenke]

That would mean things like rgb(255 40% 100%) are valid. Do we want that?

Tools like VSCode actually already support this by accident.
These support percentage values but don't check that all values use the same unit.

• some color functions already have mixed units and var() calc()
• var() and calc() can obfuscate units

These things considered I think it will help stylesheet authors if mixed units are just supported.

[svgeesus]

That would mean things like rgb(255 40% 100%) are valid. Do we want that?

However, RCS already allows rgb(from black 255 40% 100%) (where the 'from black' does nothing and is not used in the computation).

[cdoublev]

Closing in favour of #8231.