First, I am not a lawyer.
Second, I never thought about it this way, but I am pretty sure no license is needed.
If you are talking about automatic completion provided by the Developer Tools when you enter a CSS property or value, or if you are talking about the computed styles of your own website, then there is no reason there would be a license associated with it.
If you are talking about computed styles of a website that you do not own, then here you might have a problem - you are basically copying work of others, in which case, I would guess you need a permission to copy their implementation.