[selectors4] Using the complete profile in non-interactive media?

Currently, Selectors 4 [1] says: 

> CSS implementations conformant to Selectors Level 4 must use the ‘fast’ profile for CSS selection.

I'm not sure what this leaves besides the Selectors API, but shouldn't UAs use the complete profile for non-interactive media as well, such as print? The performance issues are not present there, right?

Also, why is this a "must" and not a "may"? With this wording, if a UA finds a way to support the complete profile for CSS Selection, it will be a spec violation! Or am I missing something?

[1]: http://dev.w3.org/csswg/selectors4
[2]: http://dev.w3.org/csswg/selectors4/#profiles


Lea Verou
W3C developer relations
http://w3.org/people/all#lea ✿ http://lea.verou.me ✿ @leaverou