But how would you then download the edited image? Either browsers support taking screenshots via an API (no browser extension) or you would need to render it on the server side again. Maybe I am missing something?
[1] https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API/...
I'm pretty sure the applied css effects should carry over to the canvas.
Recently, I've been looking more into WebGL and what's possible with GLSL shader scripts for image warping (for VR) and image processing/effects. see shadertoy.com etc.