New principle: Incorporate from existing CSS Design Principles

[fantasai]

CSS Design principles have been articulated in a few places in the past, it would probably be a good idea for the TAG to evaluate which ones of these should be incorporated into the Web Platform Design Principles.

[martinthomson]

The next step here is to work through the list of principles from the various CSS documents and capture those that we want to take in new issues (sub issues, perhaps). I suggest that the roc principle is great, but probably not something we can put in our specs. Others might already be ones we have (CSS 2.1 has backwards and forward compatibility principles, which we already address). So the task is finding and listing out where we have gaps.