See also output_xhtml.h:
typedef std::deque<html::StartTag> TagDeque;
///
typedef std::vector<html::StartTag> TagStack;
/// holds start tags until we know there is content in them.
TagDeque pending_tags_;
/// remembers the history, so we can make sure we nest properly.
TagStack tag_stack_;
///Why the difference ? And why the confusing Deque/Stack/Vector terminology ? Vincent
