[css-values-5] Receive comma-separated arguments in `*-progress()`

[cdoublev]

All math and mix functions receive comma-separated arguments whereas progress functions receive keyword-separated arguments.

What is the reason for this difference? Does this makes up for the inconsistent and more verbose syntax, which is somewhat unusual for a CSS function?

Nit: since progress functions can be used in math functions, which are internally represented with a calculation tree, you can no longer assume that any operator node representing a function (assuming progress functions are represented as operators) should serialize with its values joined with , ; this information must be preserved.

[fantasai]

Thanks for pointing this out! I had noticed it and then forgot to follow up. I don't mind too much if mix() and progress() differ from the rest of the math functions, but those two have a very similar structure and should probably be consistent.

[tabatkins]

I agree that mix() and the progress functions should be consistent in their syntax, since they're doing such similar things. mix() doesn't have any choice - because it's using <whole-value>, the only choice for its separators are (upgradeable) commas; keywords are absolutely no-go. So that suggests we should shift the progress functions over to just being comma-separated arguments.

[fantasai]

Apparently I did notice, and filed this as #10489 :P Closing as a dup.