After a bit of research, there really isn't much in the way of distraction-free writing apps that work in a browser. The most popular (Typora, commercial and FocusWriter, FOSS) are native. Probably this is an entirely reasonable thing. Currently I'm looking at notes apps. Standard Notes seems nicest, but it's a bit of a pain to get running. Reasonable alternatives are Trillium, Wiki.js, BookStack, WriteFreely and the various NextCloud notes add-ons.