> but PoW scales
Not if the honest party is doing it in a browser: The same computer can so any POW so much faster in C than any amount jf JS and WASM that it will never ever ever be a contest.
> becoming much more obvious and easy to block, or they have to use massive amounts of compute.
If you believe this, please contact me: I think compute is free[1] and can probably help you out.
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Can you not design a PoW that is most efficient in a browser? Don't brute force hashes like Hashcash/Bitcoin, do something similar to RandomX instead but in JS. Browsers ought to run the fastest JS interpreters already so if interpreting JS becomes the bulk of the work, that attack might not work. Maybe even involve the DOM or whatever else makes sense.