I built a child theme locally using Toolbox and was in for quite the surprise when I uploaded it to a server and ran it through crossbrowsertesting.com. My header was collapsing in certain browsers causing the navigation to be unusable. A few hours of headaches later I finally realized that the header element isn't set to display: block anywhere in the Toolbox CSS (that I can see). I made the change and it seems to be fine for the most part now (Firefox 3.0.6 and Ubuntu 8.10 don't quite get it right).
Why aren't all the HTML5 elements set to display:block? That's one of the first things I learned about when starting into HTML5.