https://w3c.github.io/media-source/#mediasource-detach

"The following steps are run in any case where the media element is going to transition to NETWORK_EMPTY and queue a task to fire a simple event named emptied at the media element. These steps must be run right before the transition."

Blink also detaches in the fetch algorithm failure steps, when networkState reaches NETWORK_NO_SOURCE. Should this be part of the spec? I'm not sure it matters much, but things may be easier to understand/explain if the resource selection algorithm is the only thing that invokes "detaching from a media element."