I also noticed that they are on Windows 7. Which, honestly I didn’t think was possible.
It kind of clashes with your HTML/CSS only ethos, but considering the target audience (monastics with ancient tech, global low resource communities) it might be a kindness to add in some kind of fall back. Perhaps you could even just do something that wasn’t a full replacement for browsers that don’t support it, just not quite so broken.
¯\_(ツ)_/¯