For things missing/hard in WebGL, is it performant enough to rely on the browser compositor to add a layer of text, or a layer of svg, over the WebGL? I have some zig/wasm stuff working on canvas2D, but rendering things to bitmaps and adding them to canvas2d seems awfully slow, especially for animated svg.

Ah man, I'm still looking for a general canvas drop in replacement that would render using webgl or webgpu if supported. Closest I've found so far is pixi.js, but the rendering api is vastly different and documentation spotty, so it would take some doing to port things over. Plus no antialiasing it seems.

You don't need multithreading to get concurrent asset streaming, a completion callback or async-await-style code will work too (after all, that's how most Javascript web games load their assets "in the background"). Also, browsers typically restrict concurrent download streams to about 6 (the exact number is entirely up to the browser though) - so you can have at most 6 asset files 'in flight'. In the end you are still limited by the user's internet bandwidth of course.