Was going to try out the browser demo, but then this happened.
What device are you on? This is apparently a common error on apple devices for the version of ren.py that the game is running on, especially Safari & iOS. There is a build you can download for apple computers, but iOS is not supported at this time.
If you are not on iOS, I definitely still recommend trying one of the downloadable builds as you should not run into this error there.