Macaw is a forthcoming Mac application for designing pages for web browsers that are ready to use as web-native code. The app runs natively in html and css, and provides excellent export to clean html and css.
There are some great optimizations for nudging/pudging on a grid, and for setting properties that make sense for css—thinking of positioning and dimensions.
The outline view is a nice way to browse through all the groups in the document and adding container tags and/or classes for semantic html publishing. It's well matched to how you'd think in html. It leap-frogs the heavier prototyping tools, but it's limited to creating static pages, like comps, for now. I'm not sure what the graphics output options will be.
The html export is clean, maybe cleaner and more efficient than what I would write because it's a robot. Look really interesting, and is yet another approach to designing more directly for the browser.