What about wireframing in HTML itself. Like http://jetstrap.com lets you do with Bootstrap? Fluid, exportable and could probably be versioned pretty easily. Your post also reminds me of Jason Santa Maria's post about "actual web design tools." edited to add link to JSA's article: http://v4.jasonsantamaria.com/articles/a-real-web-design-app...
Currently I do a decent amount of wireframing in the browser. I find this too slow though, especially when dealing with complex interfaces that must work on multiple devices. Trying out a simple idea can mean writing a decent amount of CSS.
It would be quicker if I could just draw a fluid box on a page somewhere.
It would be quicker if I could just draw a fluid box on a page somewhere.