Currently nsIChannels get a certain priority level based on what part of the page load created it (img vs script, etc). We should pass this priority value through in a pass-through service worker fetch(). There is an open spec issue to expose this on FetchEvent.Request.priority, but in the meantime we can implement it as a hidden internal value that gets passed through. https://github.com/whatwg/fetch/issues/436
Note, there is a spec PR to pass the priority internally as suggested in comment 0. See: https://github.com/whatwg/fetch/pull/785
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