> 1. An unknown element encountered within HEAD shall be interpreted as an
> implicit termination of HEAD and shall be interpreted according
> to the content model:
That heuristic will break on future extensions like <STYLE>,
which can appear in the <HEAD> and has content.
Oh? How would STYLE break this heuristic? STYLE only has #PCDATA content.
It won't have any possible subelements which might be treated as unknown.
Glenn