At this point, 'isExternal' or 'isXml' is FALSE.
Search for jsp:root action, in order to determine if we're dealing
with XML or standard syntax (unless we already know what we're
dealing with, i.e., when 'isExternal' is TRUE and 'isXml' is FALSE).
No check for XML prolog, since nothing prevents a page from
outputting XML and still using JSP syntax (in this case, the
XML prolog is treated as template text).