[css-color-4] Computed/used value of color doesn't specify how to serialize.

[AmeliaBR]

The section "Resolving Color values" in CSS Colors 4 makes it explicit that computed & used values for color should use rgb or rgba notation.

But it doesn't say whether to use the (0-255) format or percentages. Quick test suggest that integer format is preferred by browsers.

On high-color-depth monitors, converting all colors to integers could mean a loss of data, if the numbers were always rounded off to a nearest integer. But since the latest specs allow decimals in the 0-255 syntax, maybe the rounding could be left as implementation-dependent?

[svgeesus]

Agree this should be specified.

Integer format is historically more popular, but it also assumes an 8 bit per component model. The introduction of wide gamut displays means that higher bit depths are becoming more common. Canvas is also shifting to allow more bits per component.

Thus, CSS Color 4 is tending to prefer a bit-depth agnostic 0 to 1 (or for rgb, 0% to 100%) I would prefer to see the % notation.

The introduction of wider RGB spaces, ICC colors, Lab, LCH also means that computed and used values, and serialized values, need to be specified there too.

[tabatkins]

The rgb() numeric args are numbers now, not ints, so they can handle any bit depth with no problem. Since impls have historically serialized with the number syntax, we should probably preserve that in the serialization rules.

[svgeesus]

Agreed. Lets clarify in the spec that the rgb(percentage form, with optional alpha) is the serialization. At least, as long as the working colorspace has it's initial value of sRGB.

[AmeliaBR]

rgb() with percentages would be a breaking change from existing implementations.

I agree with Tab, it is better to standardize on rgb with arbitrary precision numbers, since this simplifies to integers if the input is rgb integers or hex codes (maintaining compat), while still allowing for higher precision value or values beyond the sRGB gamut if the specified value uses a different format.

[svgeesus]

The change from ints to numbers in rgb() could easily be missed, so I think this needs an example in the spec.

It also directly contradicts CSSOM, which is now fine as we resolved today to remove the serailization of RGB from CSSOM and consolidate it all into CSS Color 4; so that text can now have the precision test built in.

rgb(146.064 107.457 131.223)
is now valid, and equal to
rgb(57.28% 42.14% 51.46%)

Given the conflict with what CSSOM used to say, this needs tests too.

[svgeesus]

Examples have now been added to the spec, serialising for all the sRGB-based ones to `rgb( )' with suppression of trailing zeroes, and of trailing decimal if all the fractional part is zero, and suppression of fully opaque alpha, and no commas. Not sure about the commas though for compat.