[css-color-5] Flesh out adjuster normalization logic for color-mix() further

Author: LeaVerou

css-color-5/Overview.bs

@@ -124,8 +124,10 @@ Arguments are normalized as follows:
 it is expanded to all adjusters available for the given [=colorspace=] plus alpha.
 2. If a percentage is omitted and there are no adjusters present on any color arguments,
 it defaults to 50% (an equal mix of the two colors).
-3. If only some adjusters are present, the remaining ones default to 0% (i.e. the corresponding components are taken from the first color).
-4. If the sum of corresponding adjusters across all colors is not 100%, percentages are scaled proportionally so that the sum is exactly 100%.
+3. Adjusters that are not present on **any** color default to 0% (i.e. the corresponding components are taken from the first color).
+4. Adjusters that are only present on one of the colors default to <code>100% - <i>p</i></code> on the second color,
+where <i>p</i> is the percentage of the corresponding adjuster in the other color.
+5. If the sum of corresponding adjusters across all colors is not 100%, percentages are scaled proportionally so that the sum is exactly 100%.
 
 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.