2D shadows is kind of a niche topic, but understandably so. I don't think this effect is any better than just slapping a simple circular gradient light on top, the extra complexity is not worth it in my opinion. But I like wasting my time implementing stupid shit so ahauwheuahawuehawe.
This is basically a WebGL version of this article on how to implement shadows
Just run a local web server as this thing doesn't have any dependencies and is written in pure WebGL (Also makes the code kind of long and annoying but whatever)