[css-images-4] Restriction on `<percentage>` values in `stripes()`

[cdoublev]

<percentage> is currently restricted to [0,100] in stripes() otherwise the function is invalid:

<color-stripe> = <color> && [ <length-percentage> | <flex> ]?

• <percentage [0,100]>: [...] Only values between 0% and 100% (inclusive) are valid.
• <length [0,∞]> : [...] Negative length values are invalid.

I agree with #9516 (comment) that values outside this range are meaningless.

But then I am not sure why they make sense in <alpha-value> or <brightness()> (for example), which produce different animating behaviors (because <alpha-value> is clamped at parse time).

However my main concern is <length-percentage> expanding to <length [0,∞]> | <percentage [0,100]>, which is not great for grammar driven parser, therefore I suggest this change:

- <color-stripe> = <color> && [ <length-percentage> | <flex> ]?
+ <color-stripe> = <color> && [ <length [0,∞]> | <percentage [0,100]> | <flex> ]?

Note that <length-percentage [0,]> is not problematic because [0,] can be "inherited" during expansion.

[Loirooriol]

Yeah, I don't think it makes much sense to forbid 101% but allow calc(1px / 0).

And as you say the grammar uses <length-percentage>, it's not clear how the restriction applies to e.g. calc(101% - 100px).

But rather than dropping <length-percentage>, I would just drop the restriction and define it correctly.

[cdoublev]

I do not think calc(-1% + 1%) should be an invalid <percentage [0,100]> and:

Parse-time range-checking of values is not performed within math functions, and therefore out-of-range values do not cause the declaration to become invalid.

https://drafts.csswg.org/css-values-4/#calc-range

But I wonder why some values are invalid when out of range, instead of being consistently clamped at computed value time, so I am also fine with dropping the restriction. Cc @SebastianZ, who may disagree.

[SebastianZ]

Clamping the values instead of disallowing them seems fine to me. Question is whether it should happen at computed value time as suggested by @cdoublev or already at parsed value time to be consistent with <alpha-value>.

Sebastian

[Loirooriol]

I don't think this needs to be consistent with <alpha-value>, which is a pure percentage, here percentages resolve against a length and have type «[ "length" → 1 ]» so it's an entirely different situation.

So I would just rely on

The total width is defined by the context in which the stripes() function is used. If the sum of the stripes is smaller than the total width, the paint line is transparent black for its remaining length, as if a final transparent argument were given. If the sum is larger, any stripes or portions beyond the total width are truncated.

I don't see the need to truncate individual values at all.