[css-typed-om-1] currentColor is not the only non-absolute color

[svgeesus]

In 6.7. Serialization from CSSOM Value, several times we read:

If the value is the currentcolor keyword, return "currentcolor".
Otherwise, return the result of serializing the <color> value.

That isn't the only case for non-absolute colors. It would be better perhaps to reference CSS Color 5 absolute color whose more comprehensive definition is:

An absolute color is a <color> whose computed value has an absolute, colorimetric interpretation. This means that the value is not:

• currentColor (which depends on the value of the color property)
• a <system-color> (which depends on the color mode)
• <light-dark()> (which depends on the color mode)
• <device-cmyk()> (which has no colorimetric basis)

Nor are any of those values used inside <color-mix()> or in relative color syntax.

[svgeesus]

@tabatkins would you like a PR for this or should I just make the edit directly?