[csswg-drafts] [cssom] Define "canonical order". (#5741)

emilio has just created a new issue for https://github.com/w3c/csswg-drafts:

== [cssom] Define "canonical order". ==
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.

cc @fantasai @tabatkins 

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/5741 using your GitHub account


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