Actually I expect it would work well for math. I'm a 40-something banker who's taking General Chemistry at a local college (to ward off brain-rot). And I got an A last semester, so I think I'm doing it right. I do copious outlining of the textbook, which is much more detailed than the lectures. The online material consists of the slides the professor uses in the lecture (not very useful in my opinion, but good for an occasional quick review), as well as detailed solutions to all the problems presented in the recitation section (different from the ones in the textbook-based homework). If you were trying to scribble down all the solutions as the prof writes them on the board, you wouldn't really be able to pay attention to what's going on. However, it's very useful to have copies of them for review, as they tend to be similar to exam questions.
I think it depends on the class. My students are high school/remedial math freshman students. The school websites I've looked at weren't too encouraging. These kids lack the type of fundamentals that you and I had mastered by the 5th or 6th grade. Their teacher's websites don't really cover that kind of material.
Now I would have loved that kind of website for my college math and science classes.