Dedicated browser would be good, and probably could use WebKit or Servo but what about all the functionality around just displaying sites? I don’t know which of them are included in rendering engines and which are not, but implementing things like developer tools, indexeddb, calendar for date input, video playback, webrtc… isn’t that too much?
Perhaps the only way is to drop browser support and just limit to native apps? There is one API less to maintain 
When somebody wants to use HTML/CSS/JS, they always have Electron. What’s the advantage of web apps that we so desperately need? One post on the topic from a month ago: