[css-values] Clarifying serialization of negative zero and expression simplification

Recently I was discussing with @emilio how signed zeroes should be serialized in calc expressions, and couldn't find a clear answer in the spec.

For example, an expression like max(1% / -infinity, 0%) which should simplify to max(-0%, 0%). Without knowing if the percentage basis is positive or negative, it can't be simplified further since zeroes of different signs are not considered equivalent in min/max functions.

It looks like currently Chrome and Firefox serialize this as max(0%, 0%) while Safari does calc(0%). I'm not sure which one of these is correct. max(-0%, 0%) is perhaps another possibility.

I found two parts of the spec that look to be relevant.

The serialization steps mention infinities and NaN but don't have any explicit instructions for negative zero. The numeric value should be clamped to the allowed range, but the allowed range may include negative values.

10.9.1. Infinities, NaN, and Signed Zero mentions that if a top level calculation simplifies to a negative zero, it should be "censored into a standard representable value".

Could that apply here? The example expression doesn't fully simplify, but the only two possible resulting values would be equivalent to 0%, so calc(0%) seems reasonable in this case.

But I'm not sure if that would work if there are additional arguments. Would it also be ok to simplify something like max(-0%, 0%, 1%) to max(0%, 1%) if it's the top-level expression? Does it make sense in general to eliminate duplicated values? Safari serializes max(1%, 1%, 1%) as calc(1%), while Chrome and Firefox keep max(1%, 1%, 1%).

Start by reading the CSS Values and Units specification sections on calc serialization and infinities, NaN, and signed zero. Compare the issue's max() and duplicate-value examples with the reported Chrome, Firefox, and Safari serialization; done means reaching and documenting an agreed serialization rule in the spec.