To all the people saying "it's dangerous to add concurrency to javascript", javascript has already workers, shared array buffers and atomics. It's entirely possible today to start two or more workers, pass a shared array buffer via a message and then write concurrently on the same buffer forfeiting message passing and synchronizing only using atomics. You can even do lock less data structures, see for example https://greenvitriol.com/posts/lockless-allocator. That's what you do when you write high performance Web apps.
This proposal only adds lightweight threads sharing memory by default, but it's by no means the first and only way to do low level concurrency with javascript.
So you add threads to allow a shared objects graph with a side effect of more frequent stop the world. Then you fight stop the world by resorting to low level memory management. But if you go level why not allocate a shared array buffer and share it across workers? Something you can do today without threads.
