Showing and hiding details content

I hope that my question doesn't seem so trivial.
The spec says, for <details> element, that "When the element is created, if
the attribute [@open] is absent, the additional information should be
hidden; if the attribute is present, that information should be shown",
without any further detail on how it is shown or hidden (sorry for the pun).
Afterward, in the "Rendering" section, the working of <details> is
described in terms of shadow DOM and it is stated that "when the details
element does not have an open attribute, this second container is expected
to be removed from the rendering". Is this equivalent to say that such
"shadow container" will have the CSS property "display:none"? And will
authors be able to access the rendering of such element, so that they can
change e.g. the appearance of the "open/close" indication (of course in a
far future when shadow DOM is reliably implemented)?

Aside, images in
http://www.w3.org/TR/html51/semantics.html#the-details-element are broken.