It also helps that working on a browser engine is "easy" in the sense that you are developing to a known target and there are increasingly decent test suites available. It helps a lot with keeping motivation alive since there's a persistent drip feed of low-hanging fruit and it'd easy to quantify your productivity and you don't get super stuck, vs. having to innovate or greenfield stuff. Teams of skilled volunteers excel at this, and makes for fairly easy contributor recruitment to feed that team. Making a browser engine is almost the ideal case.

The breadth of the task is staggering, but there is a real shot at posting the necessary endurance given the circumstances, being thrifty and given time.

Before you reach for the downvote button for talking out of my arse and calling what the Ladybird folks doing a (qualified, specific) "easy", when we at KDE created KHTML which later begat WebKit, for many years - kling might remember! - our IRC channel topic was "which mozilla bug do we emulate today?", so I've seen it happen once before! Ladybird can succeed; consider contributing.

I guess it makes sense. Almost nothing could be easier to dog-food, and if you get stuck you can always read how Blink, WebKit, and Gecko implement something.