I'm building a mobile app using flutter, which has so far been an excellent experience. They've announced development of a web version of it - hummingbird (https://medium.com/flutter-io/hummingbird-building-flutter-f...) which I'll be keeping a close eye on. Not only is it a fundamentally productive toolkit to use, but sharing both business and UI code between all frontends (desktop, mobile and web) would be wonderful.
I've also heard some positive things about elm-ui (https://github.com/mdgriffith/elm-ui), which is on my todo list to investigate.