Mathworld is good to search for relatively wacky stuff (As is Wikipedia - you can sometimes find niche results squirrelled away in the depths of a big article), but as a resource for learning it's not great.
It is true that outside of Calculus (through to real and complex analysis) and Linear Algebra the density of good teaching material online decreases
It is true that outside of Calculus (through to real and complex analysis) and Linear Algebra the density of good teaching material online decreases