I've never heard of mdn before this thread somehow. But w3schools is perfect for looking up the things I tend to look up (mostly CSS or HTML attributes) and it's really easy to parse. Plus it's always the first google result for me.
I've been developing websites since 1996 myself, and MDN hasn't really been on my radar for an html reference. I did use it extensively back in 2009 when I was doing work with <canvas>, and back then MDN didn't really have a general html reference. I think it's only in the last couple of years that MDN has really developed their reference documentation.
So, if you've always used MDN you really must be a young whippersnapper :)
I think MDN has an advantage in that they've only really been around since HTML5 has become standardized, whereas w3schools has had to deal with the changing standards of html4/xhtml/html5.
"I think MDN has an advantage in that they've only really been around since HTML5 has become standardized"
=> HTML5 is not a standard. Nowhere near so. Whatever you mean by that, MDN is as old as Firefox anyway.
"...whereas w3schools has had to deal with the changing standards of html4/xhtml/html5."
=> These differences are 99.99% additions, so don't impact documentation that much. In any case, documentations is more relevant is it documents implementations, not standards (which come after implementation most of the time, not the other way around until recently)
