I don't really get it, maybe my phone is just old but I saw exactly 3 fonts on this page: serif, sans, and mono. None of the other fonts are available on my phone so each category looked identical. Is this experience better on other devices? To me this was not a useful demo because it reinforced the fact that you need to ship fonts if you want users to see that specific font.
I assume you're on Android? That's the weak link when it comes to system fonts, it only ships with a handful so many of these stacks end up looking identical in practice. If you go through to the GitHub page there's screenshots of how each stack gets rendered on each platform.
If you're not happy with what Android offers then a possible compromise would be to make a hybrid stack which tries Windows and Apple system fonts first, then falls back to a webfont if those aren't available.
Hah, I ran into the same issue on Linux. I forgot that I only install a couple fonts on my machines (noto, cascadia, and adobe-source-*) so I was also looking at them thinking "Why are these all the same? Even hand-written looks like a normal typed font". Thanks for the suggestion to go to the github for the screenshots: https://github.com/system-fonts/modern-font-stacks
Yeah I'm on android. I guess the hybrid approach offers a couple benefits, thanks for the reply.