Your comment on the width is well-received. I doubt many people are going to use this to create a mockup wider than 960px, so when I implement the grid I think I'm going to limit its width. I'd appreciate everyone's thoughts on this.

Ctrl C/V : Will do.

Re layers: Can you give me an example (share your mockup link) where that would be useful? I worry that it would complicate things more than they need to be.

Can you elaborate on the shift key? How does that work?

Eventually I'd like to implement a rich text feature where you can format the text however you want. Currently, as you noted, an element's font applies to all the text within it.

Thanks for the feedback. Keep it coming.