[css-values-4] Allow more simplification of operator nodes

[cdoublev]

Maybe returning a result in its canonical unit should not be mandatory?

4. If root is an operator node that’s not one of the calc-operator nodes, and all of its calculation children are numeric values with enough information to compute the operation root represents, return the result of running root’s operation using its children, expressed in the result’s canonical unit.

https://drafts.csswg.org/css-values-4/#simplify-a-calculation-tree

It prevents simplifying min(1em) as calc(1em) (related: #9559), clamp(0em, 2em, 1em) as calc(1em), round(1.1em, 1em) as calc(1em), etc.

Chrome/FF currently simplify min(max(1%, -1%)) as max(1%, -1%). There does not seem to be any corresponding WPT tests. Support for other math functions is still low so I have not tested them.

[tabatkins]

In general, I have no particular opinion on how the simplification rules should work, so long as they're consistent and reasonable. I'm happy to match browsers if they're in agreement and the result makes sense.

min(1em) as calc(1em)

I'll leave this to #9559.

clamp(0em, 2em, 1em) as calc(1em), round(1.1em, 1em) as calc(1em)

It doesn't seem particularly useful to me to require this work, versus just waiting until they can actually finish the answer. (And in the clamp case, you can't do that simplification if the arguments are %s.) Are browsers currently doing so?

Chrome/FF currently simplify min(max(1%, -1%)) as max(1%, -1%)

This looks like it's probably #9559 too.

[cdoublev]

Fwiw, I do not understand why the shortest serialization principle exists for specified values. But I have read that implementers prefer to avoid preserving as many of the calculation nodes as possible, probably to save memory (and the UX), I guess.

And in the clamp case, you can't do that simplification if the arguments are %s. Are browsers currently doing so?

No. I guess I cannot simplify them when one but not all have a negative value, right.

So it seems that everything is already well specified! Sorry. =)