Browsers display content that follows a very specific set of standards. There are also suites of tests to verify your compliance with those standards. So, every browser that is standards compliant should work for the vast majority of websites in existence. Still a big lift but doable for talented team.

Now an OS without application compatibility is kind of DoA unless there is a very compelling reason to switch. Add in hardware compatibility and it gets even worse.

Much bigger hill to climb then incorporating an existing browser engine into a custom spin of a browser. Even a browser engine from scratch would be smaller than a new bare metal OS.