When you say "terms listed in red", do you mean the code snippets? Those are the only text in most fcc exercises shown in red. Most of them aren't terms per se, just markup, names of methods, etc.
Agree with the MDN suggestion (that will cover much of the stuff in those code snippets too, e.g. look up
div and you'll find info about
div elements, look up
splice and you'll find info about the
splice() array method, etc). W3 documentation is good if you want to learn the intricacies of various web technologies, but the docs tend to be prescriptive rather than descriptive, which means they cover how things are supposed to work, not necessarily how browsers implement things.