[cssom] Define "canonical order". #5741
Description
CSSOM makes references to "canonical order" in a bunch of places to refer to the order in which a shorthand expands to their longhands.
For example, for:
flex: 0; flex-grow: inherit;
Browsers serialize it as:
flex-shrink: 1; flex-basis: 0%; flex-grow: inherit;
which means that "flex" expands "flex-shrink" before "flex-basis". I don't think that's defined anywhere, and it probably should.