Last modified: 2013-09-20 06:10:58 UTC
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)
Examples for captions: https://www.mediawiki.org/wiki/VisualEditor:TestImageCaptions Examples for references: https://www.mediawiki.org/wiki/VisualEditor:TestReferenceContents
Re references: yeah, that struck me as highly likely to confuse people (and highly unnecessary) when I saw it.
The references dialog has no references button already. Not sure about image captions as their broken on my local copy, but the code is there to specify the toolbar for each dialog.
This was completed by the toolbar re-write that was merged for the 12 September release; moving bug 50459 from a blocker (which was wrong anyway) to a see-also.
