Good question! Maybe with my side project, I can shed some light on the state of OpenGL exposed through the web:

I maintain a Matrix code rain project: https://github.com/Rezmason/matrix . The effect is basically a compute shader with post-processing steps applied on top of it. It's built on top of WebGL via ThreeJS, and I can attest that its cross-browser, cross-device support is currently terrible.

That's because ThreeJS's (otherwise nice!) compute shader implementation is written on top of WebGL 1's fragment shaders, rendering to floating point buffers enabled through the "oes_texture_float" extension.

This extension's implementation has been plagued by spec mistakes, made possible (I believe) by ambiguities and tacit assumptions about differences between the speccing process of OGLES 2 and WebGL 1 that proved incorrect. Fixing it has frayed implementors' constitutions and made the API more complicated; WebGL 2 incorporates this extension's behavior into its core, but isn't widely adopted, because (in my opinion) asking a browser maker to double down on their investment in WebGL again after the aforementioned debacle is a tall order. Or maybe there are security implications. Or maybe the only Mobile Safari programmers available decided OpenGL is a weird-shaped API to expose through JavaScript, and have lost interest. One can only speculate.

Anyway! In the meantime, to do any sort of GPU compute across all major browsers (including mobile), you're way better off enabling the "oes_texture_half_float" extension instead. Half floats, or Float16s, are common in the mobile graphics sphere, but not on the desktop, where most web development takes place; it's entirely understandable why a web developer might try and use "oes_texture_float", get it working on their desktop machines, fail to see it working in Mobile Safari, assume that the extension isn't supported, and call it a day. And even if they DO find the Stack Overflow answers that'd prompt them to switch to "oes_texture_half_float", JavaScript has no native Float16Array data view, so they have to make/download a slow JavaScript implementation or spin their own float to half float converter.

This is the state of things with WebGL.