Do you have any information about GTK WebAssembly support in the near future? QT has announced this week the WebAssembly support, and provided a few impressive online demo. I would like to stick with GTK, I feel it more intuitive for me, but from a technology perspective, WebAssembly support is something to consider.
I don't think Broadway uses WebAssembly under the hood, but it accomplishes what you asked in your "run your gtk app in browser" comment.