[acjohnson55]

One correction: I don't think thread safety is the issue at all. First of all, existing Javascript may not have parallelism in user code, but it does have concurrency. This means you absolutely have to deal with race conditions and resource sharing. Secondly, web workers can't access the mutable state in parallel, because they can only share data via message passing of copied data. This is what prevents thread safety from being a concern, while still enabling parallelism.