I use transform feedback, which allows vertex shaders to send data back to the application. You could do this with textures as well, as described by pjmlp. I did this in an earlier version (https://github.com/hut/cellmade#) but it just felt SO WRONG. I was very happy that I found out about transform feedback to be able to do it more cleanly.

https://developer.mozilla.org/en-US/docs/Web/API/WebGLTransf...

> The WebGLTransformFeedback interface is part of the WebGL 2 API and enables transform feedback, which is the process of capturing primitives generated by vertex processing. It allows to preserve the post-transform rendering state of an object and resubmit this data multiple times.

You can fake it by misusing textures.

Actually there is a compute shaders extension, but it is only available in certain drivers configurations in Firefox.

Chrome only plans to support WebGPU.