when I open your link, enter into the age text field letters “AP”, letter-string combinations “2a”, special signs like “//” or spaces and try to click the button, then I get the error message (as intended). The error message disappears when I try to change the age but nothing happens to the input itself, i.e. AP, 2a, // or ” ” stay put despite my trying most keys on the keyboard. If it helps, I’m using Firefox version 86.0.1 (64-bit) on Fedora 32.
Either way, my workaround seems to do the job for this issue, which hopefully it’s unique to my setup.