Loading a more detailed version of an image as you zoom in is different from what a progressive JPEG does.
Loading a Progressive JPEG means you still unconditionally load the entire file, you just are able to show a low detail version before it is fully loaded. The last time I saw a progressive JPEG actually take time to load was when I had dialup.
1. You can terminate the loading process as soon as you're satisfied with the quality. It's just that browser don't do that.
2. The OPs JPEG-Clear proposal [1] also loads the entire file no matter what. It's literally just a reinvention of progressive JPEGs, presenting it as something novel.
