For invisible frames e.g. ones in background tabs, Chrome set a per-frame simultaneous loading limit; 3 for the top frame, 2 for sub-frames are permitted to go to the network stack. Excessive requests are enqueued and go when the previous request finishes. Thus, all requests will run eventually. Most fetches made by HTML elements / CSS are throttleable. WebSockets, WebRTC's data channel, Fetch API, XMLHttpRequest, EventSource, Video and Audio are excluded today to allow keep-alive connections.