My experience with exactly these and related terms has been almost opposite. Lately I needed to look up info on HTML and CSS a lot and not only am I constantly getting MDN results at the top, I usually get a snippet from the MDN page in the "instant answer" (or whatever it's called) box. Before searching for this stuff on DDG I never even realized how good of a resource MDN is (not that I actually needed much info on web related specs in the past).
Try logging out of your DDG account (if you have one), and searching for "html table tag" (without quotes) and you'll see what I mean. MDN is nowhere to be found. If you see it, then DDG must be showing us different results for some reason.
I find MDN and w3schools about equal, although I do find w3schools is a bit better laid out and quicker to get the info I'm looking for. MDN certainly isn't perfect...I just had a quick look for some html5 stuff (MediaElementAudioSourceNode) and the page is missing on MDN. I find that w3.org is the only place that has full specs for all the html5 stuff, but it's pretty difficult to navigate.
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.
