[Zope] start level of title tags in structured text

[danielle.d-avout]

how can we tweak the start level of the title tags in structured text: so
that when a structured text, written by an internaut for example,  is
integrated into a page, it doesn't break the structrure html of the rendered
page
in the case of plone, the rendered html can contain 2 <h1> as the
description of the page has already the tag <h1> and these two <h1> are not
necessarily on the same "logical" level.
I can't see any way to start the contents of the stx-text with, for example
a title of level 2
my google research was not fructful....
thanks