Many languages have a great stdlib already available, all they need is an improved version of the DOM glue for events and elements (find, adjust, add). It is doable, and the vast majority of script-browser interaction is merely to find and adjust the state of some DOM elements and respond to clicks etc. You could start small and build up and still have something useful, though security and sandboxing might be the hardest issues to cover. New native platform APIs on the desktop or mobile are rewritten from the ground up regularly and those are far, far larger projects (e.g. on Mac OS Classic, Appkit, UIKit, etc).
I think the problem has not been so much that this task is incredibly difficult, but that browser vendors have very little incentive to add a new language in addition to js - it's already a money and time sink maintaining a browser, so they're not going to volunteer to make more work for themselves without a payoff for the corporation paying the bills.
I think the problem has not been so much that this task is incredibly difficult, but that browser vendors have very little incentive to add a new language in addition to js - it's already a money and time sink maintaining a browser, so they're not going to volunteer to make more work for themselves without a payoff for the corporation paying the bills.