Drew has said [0, 1] that it's not possible/practical to build a browser today (enough counterexamples exist for this to be not really true) but the way his projects are going I kind of expect him to announce one in the next few years.
[0] https://drewdevault.com/2020/08/13/Web-browsers-need-to-stop...
[1] https://drewdevault.com/2020/03/18/Reckless-limitless-scope....