If you use Kotlin->JS, do you use any JS framework or HTML lib like Bootstrap?
We are using no UI-framework, because as I see it, it’s hard to find a UI framework which beats the power of a having proper typed object oriented language Instead we’ve tried to adapt Facebook’s Flux approach for the most advanced parts of UI. Otherwise we have done something similar like you in (http://www.yested.net/), but with less focus on “builder DSL” as for me that have had more a nice demo effect than real usage. However I have seen someone working on a Kotlin wrapper for React: GitHub - andrewoma/reakt: Reakt is a Kotlin wrapper for facebook's React library.
When it comes to styling (CSS) we are giving each Kotlin class extending an “Element” a CSS class name which is the same as the Kotlin class name. Example: Class com.acme.ui.Toolbar wil then have CSS class “com.acme.ui.Toolbar”.