Dictionaries as return values

The MediaStreamTrack interface have added a significant usage of
Dictionaries as output values; even Dictionaries inside Dictionaries inside
a return value Dictionary.

I have tried to parse the WebIDL spec and can find no evidence that this is
allowed by looking at the grammar. The text seems to indicate that it is
possible however. Historically (or when I looked at the WebIDL spec closely
last time) it explicitly stated that Dictionaries were only for arguments
but that text has disappeared.

Can someone clarify the situation and if Dictionaries can now indeed be
used for return values a pointer as to where this is documented?

/Tommy