I got up one morning and thought, I'm gonna learn HTML. Took me 30 mins, promptly forgot about it the next day
That's the thing :P If you don't use it you forget it. The other side of that though is if you go back to it it's like riding a bike. You only really need to learn once. After that it's all recollection.
It's not so much forgetting it entirely, it's more forgetting all of the obscure elements. I've been a hobbyist developer for almost 20 years, and I'm still looking up the syntax for certain HTML elements on most projects.
I do it often as well, that's why I was sure to put "basics" in the suggestion haha. It's kind of like Othello. Their old boxes used to say "a minute to learn, a lifetime to master"