Align Native color parsing with MapLibre Style Spec

[chrisgervang]

Motivation

Follow-up to #4434 and #4435.

MapLibre Native’s vendored CSS color parser supports a narrower syntax than the current MapLibre Style Spec parser. PR #4435 addresses the immediate four-argument hsl() issue, but several related syntax differences remain.

It would be useful to align Native with the specific CSS Color 4 subset already implemented and tested by MapLibre Style Spec.

Proposed syntax support

HSL/HSLA

• Space-separated components:
  • hsl(120 100% 50%)
• Optional slash alpha:
  • hsl(120 100% 50% / 0.25)
  • hsl(120 100% 50% / 25%)
• Identical grammar for the hsl() and legacy hsla() aliases.
• Percentage alpha values.
• Hue numbers with an optional deg suffix.
• Decimal and scientific-notation numeric values.

RGB/RGBA

• Space-separated number or percentage components:
  • rgb(255 0 0)
  • rgb(100% 0% 0%)
• Optional slash alpha:
  • rgb(255 0 0 / 0.25)
  • rgb(100% 0% 0% / 25%)
• Identical grammar for the rgb() and legacy rgba() aliases.
• Four-argument comma syntax for either alias.

MapLibre Style Spec references

The supported subset is documented here:

• Declared grammar and caveats
• HSL/HSLA parser implementation
• HSL/HSLA syntax tests
• Additional HSL conversion and parity tests
• RGB/RGBA parser implementation
• RGB/RGBA syntax tests

Relevant CSS Color 4 sections:

• HSL notation
• RGB functions
• Alpha values

Out of scope

This would target parity with MapLibre Style Spec rather than complete CSS Color 4 support. The Style Spec parser currently does not support:

• grad, rad, or turn hue units
• The none keyword
• Comments inside color functions

Acceptance criteria

• Native accepts the HSL and RGB forms listed above.
• hsl/hsla and rgb/rgba behave as aliases for the supported grammar.
• Equivalent syntaxes produce the same premultiplied native color.
• Existing invalid mixed-separator and mixed-number/percentage cases remain invalid.
• Native unit tests cover representative cases from the Style Spec test suite.