In general, "everything" is allowed (text; annotations; inline and block images; headings; lists; HTML blocks; entities; transclusions; references; etc.), except for:
Image captions
-> No lists
References
-> No references (but transclusions of {{#tag:ref|text}} get around this for one level as an out ref; eww)
Version: unspecified
Severity: minor
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=50459