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WASM threads are available in all modern browsers now, including Safari. It's very early days in terms of ecosystem but we're steadily getting there.

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The issue isn’t so much having additional threads, it’s needing two main threads. The browser has one main thread for accepting user input and then dispatches the relevant user events to the JS main thread.

The browser can either dispatch the events asynchronously, leading to the events being handled in a noticeably delayed way, or the browser can block its main thread until the JS dispatch finishes, leading to fewer UI events being handled. Either way is an inferior experience.

Also aren't service workers technically multi-threading? That's been a thing for a while in the browser now.

Technically yeah, but the threads could only communicate through message passing which isn't ideal for performance. The more recent major improvement is for workers to be able to share a single memory space similar to how threading works in native applications.

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