Discovered while testing MDL-57608.

Loading messages including youtube vids (displayed by the videojs plugin) shows this in console:

Failed to execute ‘postMessage’ on ‘DOMWindow’: The target origin provided

(‘https://www.youtube.com’) does not match the recipient window’s origin (‘http://localhost’).

It seems that, no matter of the error, the vid does look and play ok, but if possible, we should get rid of it.

There is some literature in the net about it being some http/https mismatch (iframe) and also, missing call to some "destroy" stuff in the API. Worth searching and looking if it's fixable.

Finally, please verify both the videojs' youtube plugin and the youtube (standalone) player.

Ciao