Replace percentage types by combined <*-percentage> types

[SebastianZ]

CSS Values and Units 3 defines several combined percentage data types, which clarify how the percentages will be computed in the end. These types should be used within the syntax definitions of the other specifications.

Affected specifications are:

• CSS 2.2 (see issue Add combined <*-percentage> data types from V&U to CSS 2.2 #504)
• CSS Backgrounds 3 (partly done in [css-backgrounds] Replaced <length> | <percentage> by <length-percentage> #263, see issue [css-backgrounds][css-position-3][css-transforms][css-text-3] Adjust value descriptions to reflect combined <*-percentage> values #645)
• CSS Backgrounds 4 (done in [css-backgrounds] Replaced <length> | <percentage> by <length-percentage> #263)
• CSS Color 4 (No, because percentages are not computed to numbers)
• CSS Fonts 3 (done in [css-fonts] Replaced <length> | <percentage> by <length-percentage> #265)
• CSS Grid 1
• CSS Images 3 (done in [css-images] Replaced <length> | <percentage> by <length-percentage> and <angle> | <percentage> by <angle-percentage> #268)
• CSS Images 4 (done in [css-images] Replaced <length> | <percentage> by <length-percentage> and <angle> | <percentage> by <angle-percentage> #268)
• CSS Page Floats 3 see [css-page-floats-3] Replace percentage types by combined <*-percentage> types #10079
• CSS Position 3 (see issue [css-backgrounds][css-position-3][css-transforms][css-text-3] Adjust value descriptions to reflect combined <*-percentage> values #645)
• CSS Scroll Snap 1
• CSS Shapes 1
• CSS Transforms 1 (see issue [css-backgrounds][css-position-3][css-transforms][css-text-3] Adjust value descriptions to reflect combined <*-percentage> values #645)
• CSS Sizing 3
• CSS Sizing 4
• CSS Speech 1 see [css-speech-1] Replace percentage types by combined <*-percentage> types #10080
• CSS Tables 3
• CSS Template 1 see [css-template-1] Replace percentage types by combined <*-percentage> types #10081
• CSS Text 3 (see issue [css-backgrounds][css-position-3][css-transforms][css-text-3] Adjust value descriptions to reflect combined <*-percentage> values #645)
• CSS UI 4 (doesn't have any percentages anymore)

Sebastian

[smfr]

css-transforms-1 fixed in 2644458.

[svgeesus]

I don't think this is appropriate for CSS Color 4. Percentage does not resolve to a number; actually the reverse. Numbers (which for historical reasons are constrained to assume 8 bits per color component) resolve to a percentage. Higher bit depths are increasingly common. Again for historical reasons, we have to use percentage here where really, if this were being designed now, we actually want a float in the range 0.0 to 1.0. But we can't have that because 0 and 1 would mean 0/255 and 1/255, so we have had percentages to stand in for floats since CSS1.

A <number> is equivalent to a <percentage>, but with a different range: 0 again represents the minimum value for the color channel, but 255 represents the maximum. These values come from the fact that many graphics engines store the color channels internally as a single byte, which can hold integers between 0 and 255.

[tabatkins]

That's not why Color doesn't need updating; Color doesn't need updating because there is (now) no <number-percentage> type, as we realized we can't generally handle that combo in calc().

The <*-dimension> types don't imply anything about what range in the non-percentage type maps to 0%-100%. It would be perfectly fine to have the number range [0-255] map to the percent range [0%-100%], if that wasn't disallowed by the type-resolving rules.

[astearns]

I think there was just one remaining bit in shapes, fixed in 60d3770

[svgeesus]

The remaining affected specs all have open issues now, so this mega-issue can be closed I think. @SebastianZ ?