The demo doesn't work very well for me, most of the alt combinations don't seem to do anything (alt+escape to activate works though, luckily). Might need to install the extension to try it out after all.
So far I like the precise scrolling with j/k, going up and down with G/g, jumping to a point of interest (link, image) with f, focusing input boxes with i, and yanking (copying) the focused link (or the page's url if nothing is focused) with y. The parts that work in the live demo, work really nicely.
Personally I have "search when typing" enabled in Firefox which enables navigation like this: https://preview.redd.it/jial7j6lbppz.gif?width=1024&format=m... It isn't as mouse-free as this extension aims to be, but it goes a long way already with no extensions and only a tiny learning curve (to learn some tricks like searching for the text next to a button or field and hitting (shift+)tab). For example, I typically upvote HN comments by typing part of the person's username, shift+tab+tab, enter. Looking for my own post is typically just typing the word "edit".
US, no dead keys. I do have a UK physical layout though, but aside from < and \ everything is in the same place so alt's and escape's keycodes shouldn't have changed.
The symbols as they are printed on my keyboard matches what Wikipedia calls the Linux US layout[1], but I'm pretty sure I bought this in the Netherlands and an ordinary Logitech K200 will not have custom Linux prints on it, so I'm a bit confused. There also exists a Dutch layout with the same physical keys but a different keymap[2], so that's not it.
Trip down memory lane: until Vista, Windows in Dutch shipped with the Dutch and US keyboard layouts both enabled (US was the default) and ctrl+shift as switching hotkey. People would randomly get plusminus symbols when they tried to type a colon because they had accidentally switched to the Dutch layout, which was printed on exactly nobody's keyboard. It happened so often that people would just roll with it, and I think it was generally understood among the heavier computer users that plusminus was supposed to be a colon. Sometimes, someone would reply to such messages: "press ctrl+shift and try the colon again". That person was then thanked profusely. Those were the days.
So I guess that should work then? Most of my keycodes should be the same as anyone from the USA and the demos on the page you linked work perfectly (the WASD game as well as the key combo listener that displays alt+j or whatever just fine).
(Since I can't edit my previous post anymore: ignore the mention of the escape key, alt+esc is actually the only alt combo that works.)
The search bar isn't there unless you start typing, but yes, it's similar to having the cursor in the search bar. And tab works as it usually would, it's not as if you always start tabbing from the search box.
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