We currently do it immediately. Or at least, we expose through HTMLImageElement.currentSrc what the new current request's URL is immediately, but we should be doing this (and initiating the image load itself) off a microtask. See https://html.spec.whatwg.org/#updating-the-image-data. Submitted a WPT PR to test this: https://github.com/web-platform-tests/wpt/pull/28679
<rdar://problem/77443226>