[css-page] properties that only apply within the page context

There are a few properties defined in css-page that are intended to be
used only in @page rules: size, marks and bleed.  These are all defined
with a line in their blue property definition boxes that says

  For:  @page

Does that line have any normative requirements behind it, or like an
“Applies to” line does it just informatively describe where the property
can have an effect?

I ask because I’m not sure whether size, marks and bleed should parse in
non-@page contexts and be stored on declarations.  For example:

  p { size: landscape; }

Should the property be exposed on the rule when inspecting it with the
CSSOM?

Another question is whether these properties should be usable within
@supports rules and passed to CSS.supports.

-- 
Cameron McCormack ≝ http://mcc.id.au/