We implement a subset of checks from https://html.spec.whatwg.org/#unloading-document-cleanup-steps right now, which influence whether a document is considered salvageable or not:

• servo/components/script/dom/document.rs

  Lines 2462 to 2466 in 1489de1

  	let event_sources_canceled = self.window.as_global_scope().close_event_sources();
  	if loads_cancelled || event_sources_canceled {
  	// If any loads were canceled.
  	self.salvageable.set(false);
  	};

• servo/components/script/dom/document.rs

  Lines 2102 to 2107 in 1489de1

  	// https://html.spec.whatwg.org/multipage/#unloading-document-cleanup-steps
  	if !self.salvageable.get() {
  	// Step 1 of clean-up steps.
  	global_scope.close_event_sources();
  	let msg = ScriptToConstellationMessage::DiscardDocument;
  	let _ = global_scope.script_to_constellation_chan().send(msg);

Unsalvageable documents do not get put in the bfcache. There are a bunch of WPT tests that rely on the websocket checks to force a document to be kept out of the bfcache, and we don't implement that right now.

We should use a similar model as the EventSource code in globalscope.rs for tracking all WebSocket objects so we can implement the checks correctly.