[css-color-4] Allow percent as well as float 0.0 to 1.0 in color() args

[svgeesus]

@faceless2 said

allowing a percentage as a synonym for 0..1 may be more natural, e.g. color(swopc 0 85% 70% 42%)

which is worthy of separate discussion.

The spec already allows this for several types: namely <rgb-component> and <alpha-value> in the rgba() function. So I think that it would be natural to define it as a <number-percentage>. The change still allows the full range of real numbers to be used, just in a different format. ie. 300% = 3 or 450% = 4.5, etc..

[svgeesus]

While allowing, say, 50% as a synonym for 0.5 seems unproblematic, doing so would impact #278 were @tabatkins argues for adding percentage units to the L* value. So 50 and 50% would be synonymous, except now the spec would need to clarify that here, 50% means 50 not 0.5.

Apart from that one issue, I don't see an issue with allowing percentage units as well as floats here. I don't want to hold up this issue for #278 so I'm going to agree with @faceless2 here and solve that other issue separately.

[svgeesus]

Existing tests need to be updated to also check the percentage values.

[svgeesus]

This change would also affect the gray() notation, which takes a number in the range [0,100] for example gray(50). If percentages were optional, the spec would need to specify that gray(50%) meant gray(50) not gray(0.5) which is very different.