> I've not seen any web UI frameworks that in my estimation compare to what we use at the moment.
The equivalent of CSS in the desktop world is called XAML styles... same thing except more powerful... not that C2 has any need for cascading styles within the developer environment itself.
It is possible for some of the C++ parts of the current C2 to be turned into a "user control" and hosted inside the new UI (and call code with pInvoke) so that it can get up and running much faster by transitioning instead of starting over all at once.
You are not going to get a DataVIewGrid, PropertyGrid, Dockable toolbars and windows, XML + LINQ, WebBrowser control, advanced treeviews, DataSets, on and on all in one HTML framework... You would have to build a lot of things from scratch and patchwork a bunch of libraries together to get something only "close".
Only Scirra can answer what their actual percentage of market share is between Windows and all other OSes, but if it is like the rest of the world then the market share for Apple / Linux just wouldn't justify building everything from scratch with a technology base that has zero backwards compatibility with their existing code base.