Thanks.
I tried it on my Chromebook and it didn't do it there either.
Then I realized that in both places I was running it through the itch app rather than a browser. Fired it up in Chrome, and it works just fine.
I'm not sure how to start the console in the window the itch app launches for games. I can find it in the itch app main window, but that doesn't help. Anyway, it works for me now, so I'm not going to dig any further.
(Edit: thanks also for this really fun and useful tool!)