The 'headitems' property for action=parse has been deprecated, and using it
will return a warning. If constructing a page from scratch, you should use
prop=headhtml instead. If updating an existing on-wiki page, use
prop=modules|jsconfigvars and load them into the existing ResourceLoader
code with mw.config.set() and mw.loader.using().
Also, the 'headitems' property will no longer return code related to
ResourceLoader. Loading ResourceLoader would require additional data that
is not sent by 'headitems', and given the deprecation it's not worth the
trouble to add that data.
--
Brad Jorsch (Anomie)
Senior Software Engineer
Wikimedia Foundation