[css-values-4] Serialization of Functional Notations

[fantasai]

Tab and I noticed that we don't rigorously define the serialization of functional notations, so we're proposing to add a generic definition to CSS Values L4.

The non-controversial aspects are:

• Serialize the arguments per their individual grammars.
• Commas serialize with a following space.
• Space-separate adjacent tokens (except between a value and a following comma or closing parenthesis)

The one area where we have very little agreement is

• Whether to omit optional values in specified value serialization.

Generally-speaking, we omit optional components in computed values. This is an important principle of CSSOM serialization for computed values. But we are very inconsistent for specified values--this is extra confused by the fact that multiple "serialization" sections in our specs don't distinguish between specified and computed values and just assume there's only one serialization.

[fantasai]

Specified vs computed is defined here, fwiw: https://www.w3.org/TR/css-cascade-5/#value-stages

[svgeesus]

Nice, I'm comparing with the serialization stuff in Color 4 and 5 and Fonts 4 to pick up any differences.