A lot of webdev, and really programming in general, isn't
knowing everything, but just having a good mental model of it.
The MDN Web Docs list like, everything, that can happen in a web browser. Ultimately, courses are only good for getting your footing. Most skilled developers keep the documentation handy, and get good at searching it. You don't need to remember exactly how everything works, because there's a 3-sentence explanation a few clicks away most of the time. All you need to do is remember the general shape of things, and go from there. It's fine if you don't remember all the border modes, you can look it up. It's fine if you don't remember how the various word wrapping modes work, you can look it up. The most important thing to learn is where to find stuff, and how. Google(or DuckDuckGo and friends) your problems, search the MDN, and make Cool Stuff.
Ultimately, the most important thing to do is to just keep going. Like any skill, you can only get better at this with practice. Always challenge yourself, you'll learn faster. It doesn't get easy until it's been hard for a while, but at a certain point you just get fast, and you won't even need to think about half the stuff you do now. Good Luck.