WebAssembly only helps to a small degree here. SIMD support is still too limited and real multithreading will probably never be possible due to spectre vulnerability, so you are leaving about two orders of magnitude of performance on the table.

WebGL 2.0 could be a band aid fix, but it is quite limited compared to CUDA. I'd also expect a bunch of compatibility issues.

As a side note, Photopea already uses WebAssembly.