Minor Description
It seems it happens somewhat often that users mistakenly override renderHead(HtmlHeaderContainer) rather than renderHead(IHeaderResponse). Code completion makes this a very easy mistake.
I propose renaming the somewhat-internal method renderHead(HtmlHeaderContainer) to renderIntoPageHeaderContainer or something like that. We can't just make the method package private, since it is needed by Panel, Border and FormComponentPanel to render the header sections of their associated markup.