New browser that fully supports existing technologies is probably the best option.
Similarly, there's nothing technically stopping someone from building a new scripting core in the browser itself and then implementing the JavaScript engine atop that (other than the need to support a brand-new untested framework in an environment of high-performance already-existing JavaScript engines, of course).
Similarly, there's nothing technically stopping someone from building a new scripting core in the browser itself and then implementing the JavaScript engine atop that (other than the need to support a brand-new untested framework in an environment of high-performance already-existing JavaScript engines, of course).