It is indeed an issue with the software I'm using, thank you for that hint.
I've just checked it, not with a different computer but with a different Internet browser. My default browser is LibreWolf, which is a Firefox derivate, but when I've just checked it on both Firefox and Brave, the input worked. I hadn't thought that that could even be a possibility because the inputs work on all other websites – so why wouldn't they work with a text adventure? (rhetorical question)