and the cause of your issue is as described below:
I think your issues are related to having a HTML5 doctype and using HTML5 specific html elements in your markup and css.
I assume you are aware that HTML5/CSS3 are still in development and most probably won't be officially released by the W3C for at least a few years yet. Also, the final "official" W3C HTML5 recommendations will probably, at least in parts, be different to what they are today.
Consequently, HTML5/CSS3 support varies greatly among the major browsers and most, if not all, older versions of browsers will not support HTML5/CSS3 at all.
Bottom line: using HTML5/CSS3 for commercial websites is currently done at your own risk.
Using html5/css3 atm is a bit like moving into a partially completed house and wondering why all the lights don't work since only half the wiring has been installed