see : https://drafts.csswg.org/css-color-4/#valdef-color-none

For all other purposes, a missing component behaves as a zero value, in the appropriate unit for that component: 0, 0%, or 0deg. This includes rendering the color directly, converting it to another color space, performing computations on the color component values, etc.

There is a note still in the WPT tests for relative color about none :

https://github.com/web-platform-tests/wpt/blob/master/css/css-color/parsing/color-computed-relative-color.html#L336-L340

  // FIXME: Clarify with spec editors if 'none' should pass through to the constants.
  fuzzy_test_computed_color(`lab(from lab(none none none) l a b)`, `lab(0 0 0)`);
  fuzzy_test_computed_color(`lab(from lab(none none none / none) l a b / alpha)`, `lab(0 0 0 / 0)`);
  fuzzy_test_computed_color(`lab(from lab(25 none 50) l a b)`, `lab(25 0 50)`);
  fuzzy_test_computed_color(`lab(from lab(25 20 50 / none) l a b / alpha)`, `lab(25 20 50 / 0)`);

As I understand it there are two possible cases:

• none was used in the origin color (e.g. hsl(from hsl(none 50% 50%) h s l))
• none was used in the relative color (e.g. hsl(from green none s l))

When none was used in the origin color then it should be treated as 0 because channel keywords have type number.

When none was used in the relative color then it should behave exactly the same as it does in absolute colors.

Is this correct?