The above <div> holds two images: one tiny 32px inline (base64) thumbnail in the background, stretched to full size and one huge 4200px JPEG loaded frome a file. The intention is, that the tiny thumbnail is visible as a "preview" of the full size image, while the full size image is still loading.
This worked fine in Firefox 66 (afaik) and still works in Chrome. However, Firefox 67 now shows a white background for the unloaded part of the JPEG, instead of nothing (transparent).
Submitted as Mozilla Bug 1556156