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."

Per a literal reading of the MSE spec, the media element load algorithm in the HTML spec does not qualify, as there is no transition to NETWORK_EMPTY. In Blink the "If a fetching process is in progress for the media element, the user agent should stop it." step is "implemented" by setting networkState to NETWORK_EMPTY, but per spec that doesn't happen and doesn't matter since the resource selection algorithm set it to NETWORK_NO_SOURCE.

A review of exactly which bits of the HTML spec should trigger the detaching from a media element may be in order. The "These steps must be run right before the transition." also make it such that the event order depends on the order in which HTML queues the task to fire the event and sets networkState.

If an explicit hook in HTML would help, let's ask Hixie for one.