It doesn't help that there isn't some 'central' place as a resource for Javascript (e.g. php-docs for php, python-docs for python, ruby-docs for ruby, etc). Where are the Javascript docs?
The frequency at which MDN docs are linked in StackOverflow JavaScript answers shows that people are getting more familiar with them. PromoteJS took off recently and it has helped too.
The trouble with Mozilla's docs is that they are really bad about distinguishing between cross-browser compatible JavaScript and Mozilla's own implementation. For example, the reference you linked there includes the let-statement. The only disclaimer in Mozilla's docs is that it's part of "JavaScript 1.7," which would lead you to believe it's part of a recent language standard that modern browsers are likely to support. In fact, "JavaScript 1.X" where X > 5 is code for "Firefox-only."
One might ask why all those JavaScript tutorials don't link to the standard, and why so many of them don't even mention the correct technical term "ECMAScript". No offense intended, but I guess they simply don't know any better. Although they ought to know.
For those who don't like to read technical standards, there are also nice secondary resources like the Mozilla Developer Network:
