The test is rather tricky but I think fair enough. I wanted to make the "decoding" attribute map to this property, but we'd still need the existing machinery for the src-animation stuff: https://searchfox.org/firefox-main/rev/6a252a86c5bb0fdd7a197d1c244f8a28e0827b3a/dom/base/nsImageLoadingContent.cpp#613-624 So for now left as is. I removed some redundant flag checks that are included already by the DL builder stuff. Differential Revision: https://phabricator.services.mozilla.com/D309119