Some key systems may wish to indicate "the CDM did what the app asked but has nothing to send to the server". This may also be an indication that a MediaKeySession has been initialized and is ready for use.
The keyadded event currently serves this purpose in a specific case. We could generalize it (option #3 in bug 20688).
Otherwise, we may want to define a way to indicate "the CDM did what the app asked but has nothing to send to the server". If not keyadded (or its successor), keymessage along with an "empty" message attribute makes sense.
We would then need to decide whether "empty" is an empty array or null. The former seems less likely to cause problems with apps that don't consider this case.
We may resolve bug 20688 in a way that addresses this (i.e. keyadded becomes "done").
I think this is a duplicate of Bug 19208.
*** This bug has been marked as a duplicate of bug 21854 ***