A few bits of feedback (although perhaps you just haven't got around to implementing these yet):
- On the canvas, is there some way to make it obvious why boxes are relatively positioned a particular way? For instance, when I add a new box next to the iPhone picture, it's not visually obvious why there is fairly sizable top padding. Maybe light grey shading to represent padding and such?
- Is there any reason why you wouldn't merge the 'select' tool and the 'move elements' tool into the single button (and make it the default selection)?
- Any possibility of multi or group select, for quick deletion of multiple elements?
- A 'new blank canvass' button might be handy.
Anyhow, this is some great work. I often just want to put together a very basic web interface for some personal project, and I think your tool is at exactly the right level of simplicity/complexity for that.