If you have an <img> element pointing to a long-loading resource it should prevent the window "load" event from firing until that image is loaded. However, if you cancel the load by setting the "src" to the empty string, the window "load" event should trigger and not be delayed anymore. In WebKit, the original resource load will continue to block the window "load" event. This came up investigating the failure of a CSS Transition WPT test, see https://github.com/web-platform-tests/wpt/issues/34444.
I'm adding a new WPT for this in https://github.com/web-platform-tests/wpt/pull/34463.
The WPT PR has landed, so now we should import https://github.com/web-platform-tests/wpt/commit/644aba8746b83668d5d9033cda16ae3f8bba0b3d to have the new dedicated test for this, and rebase the result for the css/css-transitions test changed in that revision as well.
The import is being done in https://bugs.webkit.org/show_bug.cgi?id=235131.