I also thought that it would be possible to develop a swing-like UI library which renders itself on an HTML5 canvas. There also would be no need for any markup.
But is there a need for this? Isn't HTML5 capable enough for these goals? I did some brain storming on a webapp similar to draw.io where there would be many Items which need to be resizable and draggable. They would need to be connected, transformed, rotated, etc., and all of that needs to be obvious from varying the borders of items, and other UI clues.