DW CS6 has 'Fluid Grid Layout' which does a responsive HTML5 website in minutes and saves you lot of trouble with writing media-queries. It's pure HTML5. Try it out.
Dreamweaver CS 6 has only two HTML5 CSS layouts, but lots of others from previous versions. Why is that?
Because HTML5 is new and still evolving as a web standard, some people are not comfortable using it yet.