Almost anyone would agree that the mayoclinic result is higher quality. It's written by professional physicians at a world renowned institution. However, getting the answer to your question requires reading a lot of text. You have to be comfortable with words like "Nonsteroidal anti-inflammatory drugs," which a lot of people aren't. Half of people aren't literate enough to read their prescription drug labels: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1831578/
The answer on yahoo answers is provided by "auntcookie84." I have no idea who she is, whether she's qualified to provide this information, or whether the information is correct. However, I have no trouble whatsoever reading what she wrote, regardless of how literate I am.
That's the balance we have to strike. You could imagine that the most accurate and up to date information would be in the midst of a recent academic paper, but ranking that at 1 wouldn't actually help many people. This is likely what's going on between w3schools and MDN. MDN might be higher quality, better information, but that doesn't necessarily mean it's more useful to everyone.