Probably the most important thing is keeping up with security fixes. I’m not an expert in web security, but my impression is that there’s a never-ending cat and mouse game between hackers and browser developers to find or patch exploits. And since browsers play such an important role in the activity of hundreds of millions… billions?.. of consumers, it has the largest possible attack surface for hackers to target.

Then there’s things like better support for web assembly (how I would love the web dev world to break the JavaScript hegemony), and the constantly shifting web standards that are meant to make websites more capable, easier to program, and more performant. E.g. things like websockets and WebRTC.