Define/link how mix percentages are normalized.

See: https://drafts.csswg.org/css-color-5/#color-mix-percent-norm

In css-color-5 I see these paragraphs:

3.1:

Percentages are normalized by normalizing mix percentages.

This doesn't link anywhere and that section stops there, never actually explaining "how".

3.2:

Normalize mix percentages from the list of mix items passed to the function, with the "forced normalization" flag set to true, letting items and leftover be the result.

Searching for forced normalization yields no results.

In css-images I did find: https://drafts.csswg.org/css-images-4/#cross-fade-function

But still unsure what forced normalization is supposed to be.

@tabatkins is there maybe another edit or commit still missing/in progress?