[css-color-5] color-mix() argument normalization

Author: LeaVerou

css-color-5/Overview.bs

@@ -118,13 +118,17 @@ the mixing is done in the ''lch'' colorspace.
 The [=xyz=] colorspace is CIE XYZ, with a D50 whitepoint, and allows computation to be done
 in a linear-light-intensity space.
 
-When a percentage is specified (without a [=color-adjuster=]),
-it is expanded to all adjusters available for the given [=colorspace=] plus alpha.
+Arguments are normalized as follows:
 
-If the percentage is omitted and there are no adjusters present,
+- When a percentage is specified without any [=color-adjuster=]s,
+it is expanded to all adjusters available for the given [=colorspace=] plus alpha.
+- If the percentage is omitted and there are no adjusters present on any color arguments,
 it defaults to 50% (an equal mix of the two colors).
+- If only some adjusters are present, the remaining ones default to 0% (i.e. the corresponding components are taken from the first color).
+- If the sum of corresponding adjusters across all colors is not 100%, percentages are scaled proportionally so that the sum is exactly 100%.
 
-Issue: If an adjuster is specified without a percentage, what does its percentage default to?
+Issue: Do missing adjusters default to 0% everywhere, or only in the second color?
+If the former, we can end up with adjusters that sum to 0% pretty easily.
 
 The meaning of the adjusters is defined by colorspace.
 For example, if the [=colorspace=] is [=hsl=],
@@ -133,9 +137,10 @@ if the colorspace is [=lch=],
 lightness means the LCH Lightness, which is the same as Lab lightness,
 but different to the HSL Lightness.
 
+Issue: If an adjuster is specified without a percentage, what does its percentage default to?
+
 Only the [=color-adjuster=]s defined for a given [=colorspace=] are available.
-For example,
-it is not possible to use the HWB [=whiteness=] adjuster unless the colorspace is [=hwb=].
+For example, it is not possible to use the HWB [=whiteness=] adjuster unless the colorspace is [=hwb=].
 The alpha adjuster is available on any color space.
 
 If the specified colorspace has a smaller gamut