There is nothing inherent about HTML5 can't do this, it's just a lack of tooling and libraries. The Windows 8 demo shows that the performance is already there.

There is nothing inherent about HTML5 can't do this, it's just a lack of tooling and libraries. The Windows 8 demo shows that the performance is already there. Look at GWT for a possible development model. It's very possible that MS will deliver something that looks very much like GWT UI Builder or Blend but targeting HTML5 and scriptable by Javascript if necessary. I'd be very surprised if MS didn't have a drop-and-drop designer solution given that this is their bread and butter.