Many new features delivers by JavaScript are backward compatible, but I don’t know how long it would be true. Canvas, VIDEO and probably many other techniques are not supported by old platforms. Could it be better to deliver new mechanism to older browser? My idea is about create minimal Firefox version for old browser. Something like brow.sh, but not for terminal. It would work in graphical web browser. I think best way is ASMJS, but maybe is there other way, like Flash or Java?
If you decided to deliver Firefox for very old browsers, you could emulate canvas (as example) by matrix of 1x1 pixels of divs. Maybe it wouldn’t be very smooth, but should works.
I also read about GTK4. GTK4 will use css to style elements more often and broadway backend wouldn’t render data on server, but instead send styled html elements. I know Firefox isn’t ported to GTK3 yet, but if you decided to use GTK4, my idea will be a lot of simpler. Just compile GTK4 with broadway backend to ASMJS, modify it and port Firefox to GTK4.
If I’m wrong, just tell me. I don’t know each technical details about ASMJS, HTML5, ECMASCRIPT and GTK/GDK/GLIB. Maybe it should introduce threads, so ASMJS wouldn’t be a good target, but better could be Java for example. Posix threads aren’t supported yet by all of browsers, so older wouldn’t support it.