The error is ambiguous, but from your phrasing, I think it is safe to say it is your use of threading.
The error specifically notes "uncaught abort(-1)" error. In my experience, this tells us that something went wrong, but you need to do more digging to find out exactly what. It actually tells you this, in the actual error. Twice.
If this abort() is unexpected, build with -s ASSERTIONS=1 which can give you more information.
Ultimately, given that you ask "Does WebGL not support threading?", we can assume that you are using threading. WeblGL does not, in fact, support threading. That is not to say this will cause a crash; the Unity manual instructs otherwise:
- Unity Manual - WebGL performance considerations
That said, you will run into unforeseen behavior, if you insist on implementing threading. Apart from the aforementioned performance cost, you will likely have logical errors arising from the resulting mismanagement of the methods you intended to multi-thread, which can quite easily lead to an abort.