I agree there is a need, but I think that need is exaggerated, writing html / css is pretty easy, its not fun but its generally nothing compared to the overall work, half the time you still have to do it manually anyway since it has to fit into frameworks etc.
I want something that lets me focus on the design aspect, it should have nice snap to grid features, ability to inspect and edit css, but it should also have a vector drawing tools, image manipulations. The current set of design tools have terrible support for most common web functionality, photoshop doesnt even have rounded borders, illustrator / fireworks / sketch / pixelmator all almost entirely ignore things that are important for designing for the web. However web based editors almost all lack any graphical and vector capabilities whatsoever.
I started on this and gave up a while ago, http://upmock.com/ and wrote about it http://arandomurl.com/2012/10/15/upmock-is-dead.html. Currently the most promising so far has been SVG Edit, however it doesnt seem particularly active nor has there been a focus on producing web designs