I would like to see Firefox move faster in implementing good web APIs, like the lazy loading of images, push for HTML and CSS improvements(like let me customize the numeric text input arrows with css instead of forcing me to use a JS library) .
Maybe the Rust rewrite will make Firefox better achitected so you could have a node version based on Mozilla technologies or you could easily use the web assembly VM in your application without having to load the full Chrome code.
The unfortunate truth is that writing a browser is almost impossible because of the complexity, IMO we need a new version of the web that breaks compatibility, make everything strict(no longer ignore errors and guess a fix for them) no more 12 ways to center an element , for the legacy web we can use the existing engines
Here's an example of website written with Rust and rendering all UI to canvas: https://makepad.github.io/makepad/ it works right now and it's very fast and smooth.
IMO tech for next generation web apps is already here and it's great.
There was also an article earlier this month or previous month from a Firefox developer about font/text rendering, in case you missed it rendering text in a way that works with all writing system is super hard.
Well, I'm joking, of course, it's not that simple. But it should be possible in the end.
Some ebook publishing platforms will perform checks and reject your epub(mobi or similar format) if is not respecting some strict rules, this makes it a pain if you have random html that works on web but you want to package it as epub because now you need to find all the bugs(or similar stupid DOM elements) and fix them as best you can.