[css-color-4] Section Ordering

[fantasai]

CSS Color 3 kept the definitions of 'color' and 'opacity' up front, and then dove into the definition of the<color> type (which runs for many pages). As a relatively new reader, I found this ordering a bit easier to work with, so filing this issue to suggest keeping that original ordering.

[svgeesus]

Agreed, this would be better.