Nitpick: I thought it didn't work because I was trying Cmd-c/v on my MacBook. I know Macs are the outlier here but it would be nice to use the native shortcuts for this.
Agree with randall that it's a good starting point. Matt1 has a pretty cool html5 mockup tool too - http://jmockups.com. Glad to see multiple people in this space.
Agreed, great start. I recommend adding a tab or tab-menu object. When I'm wireframing, out of all the basic items I have to deal with, vertical and horizontal menus (tabs, often) are the most time-consuming thing to deal with. It would be great to have this object automated in a wireframing tool.
This may be a dumb question, but what do I do with this? I imagine this is to give your web designer some ideas of what you want, but is there a way to export this into an HTML/CSS format?