My impression is that the "default" approach is not to pollyfill but proxy the calls to the main thread. They have a post about how they are (ab)using synchrono

My impression is that the "default" approach is not to pollyfill but proxy the calls to the main thread. They have a post about how they are (ab)using synchronous XMLHttpRequest calls to block the worker while proxying the request to the main process with postMessage https://dev.to/adamdbradley/how-partytown-s-sync-communication-works-4244 https://dev.to/adamdbradley/how-partytown-s-sync-communicati.... I suspect that you may want to pollyfill APIs for performance but I think for many use cases just that shim to the true API should work well enough.