The tool is meant to allow to quickly create mockups of interfaces in order to communicate ideas.
It is not meant for pixel perfect mockups nor for complex interface mockups involving several screens and hundereds of elements.
The target group for now are interaction/interface designers or design-aware developers who take part in open source projects and need to communicate their ideas. An mockup is worth a thousand words in this case. More Info »
You can just save the webpage with your browsers save function (e.g. see here for Firefox) . In the dropdown in the saving dialog, you can select something like »complete webpage« and »only HTML«.
If you save the »complete webpage« , you retain interactivity (you could zip the files, send it to someone and that person could just continue working) .
If you save »only HTML«, you still save a file containing your mockup wich you can view in any browser.
You can also just do a screenshot (in case it is more a throw-away mock anyway) using whichever tool you like.