I am used to Zen so I've used the HTML5 responsive theme as a base for my subtheme. I haven't made use of the left sidebar and changed the layout to 70% content and 30% sidebar. All of the Zen CSS files are untouched, everthing is overriden in one CSS file.
The site is intended to be fully responsive but I still haven't resolved the frontpage slideshow in resolutions below 1024px wide.