But W3Schools also carries some of the most easy-to-use indexes of pretty much everything related to web development. Can't remember the exact name of that CSS property and its possible values? Can't remember the order of arguments of a JS string/array function? Go to W3Schools and browse their lists, get the answer in 5 seconds.
Of course, there's Mozilla Developer Networks, too. But MDN's CSS Reference [1] is just an alphabetical list of CSS properties with no explanation. W3Schools' CSS Reference [2] is neatly categorized, and each entry has a short explanation next to it. Very useful when you're trying to remember, for example, "word-wrap: break-word", which is not the same as "word-break: break-all".
[1] https://developer.mozilla.org/en-US/docs/Web/CSS/Reference
[2] http://www.w3schools.com/cssref/default.asp
W3Schools is good ol' Web 1.0 stuff. Just some lists and a bunch of links. No need to ask somebody and wait for an answer. No need to watch any annoying videos. No need to follow an animated tutorial designed by someone who thinks they know how a person should learn programming. W3Schools is like an old dictionary. A dictionary isn't particularly helpful when you're trying to learn a foreign language properly, but it's quite handy when you're face to face with a foreigner and you need to think up a word for "toilet paper" quickly.
I use W3Schools when I need to remember something quickly, and MDN when I need to learn more deeply about it. When I write CSS, I always keep W3Schools open in one tab and MDN in another tab. I wish somebody would link W3Schools' lists to MDN's details. That would make the best of both worlds.