This gives me a silly idea for an "accessibility" mode, where absolutely everything on the page is invisible to sighted people, but clearly, and perfectly readable to screen readers etc.

I did some professional services work years ago, very early in my career for a public-sector client that wanted accessibility features given absolute care and attention.

It really gave me some perspective and I've tried to be conscious of it ever since; though I'm purely back-end nowadays so it doesn't apply as much.