The readme explains how the CSS :active selector is used to detect button presses.
Well, that's really only technically correct, since the strategy here is to replace the whole page with a new version of it on each virtual keypress, by using CSS to hide the previous HTML and then receiving the new updated version of the web page's content. The only tricks is to use a chunked content-transfer to have this all happen over a single HTTP request, so yes technically the page does noes reload, but it's pretty much the same thing as if it did.
Really cool trick nonetheless! And more importantly fun trick, and fun read :).
Edit: actually this should work even without mixed replace.
There’s no problem with either listed approach, GP just seems to consider a method without page reloads as adhering to a harder constraint.