Please see https://github.com/whatwg/html/issues/7383 and https://github.com/web-platform-tests/wpt/blob/c580e7a20c40a6c4f44f1fb993969a673eab8e57/html/semantics/embedded-content/the-img-element/image-base-url.html#L38-L40, which is the pre-condition assertion that WebKit fails before the meat of the test.
<rdar://problem/106515784>