The complaints about these sites is usually that (a) they're literally wrong just often enough that you don't want to rely on them, which defeats the point of a reference, or (b) they try to "teach" things that are not recommended best practices. With w3schools in particular, there's the added "ick" that they try very hard to make it look like they're an offical W3C source, and sell scammy certifications based on that confusion. They also provided (past tense, because I haven't looked at it in years) some very questionable practices with PHP that would lead to trivially insecure code.

cppreference.com is what I usually use for C++. It's well organized, well cross-linked, and uses exact standards language whenever possible. It's also very good about showing the differences between successive versions of the C++ standard libraries, which is invaluable if you're working with any sort of legacy code and wondering why something looks funny.

For HTML5/JS, MDN (Mozilla) was my default when I did front-end, but it's been a little while. I'm sure Mozilla is still great, but there might also be something easier to traverse.