[css-color-4] Undefined computed & used values

[svgeesus]

originally reported by @tabatkins as an inline issue

The above defines values that are already implemented based on the most interoperable behavior.

We still need to define how newer syntaxes work:

• comma-less with slash separated optional alpha variants of ''rgb()'', ''rgba()'', ''hsl()'', ''hsla()''
• comma separated with optional alpha variants of ''rgb()'', ''rgba()'', ''hsl()'', ''hsla()''
• ''device-cmyk()''
• ''hwb()''
• ''lab()'' and ''lch()''
• ''gray()''
• ''color()''