new Image() must load, and must report

Box construction is what fetches images, and it only walks the document. An Image the script never inserts was therefore never fetched at all, so neither load nor error could fire. html5test starts four such probes (webp, jxl, avif, heic) as background tasks and waits for all of them -- forever.

Every step logs a breadcrumb first, so a crash names the operation that caused it: the last IMGLOAD line in NSDBG.TXT is the one that did not come back.