I don't have an account. You're right, for this particular query there's no MDN in results. I just tried "css box-sizing" (the first thing that popped into my mind) and MDN is the 3rd result. The quick answer thingy shows SO questions. So it can definitely be a hit or miss thing.

I agree with what you write about w3.org, it's almost painful to navigate when you just need a quick reference.