And when the dimensions aren't defined, then progressively encoded JPEGs don't offer an advantage either, because with both progressive and non-progressive images, browsers reserve space for images as soon as their size is known, i.e. when the file header is received.
Agreed. But would you say that progressive jpegs don't offer a visual advantage in this case? Imagine if a photo has a caption below it: baseline starts rendering far away from the caption, but with progressive we'll get the caption in the correct place without a big gap between the caption and where the photo is rendering. In the case of baseline, the photo will draw to "meet" the caption.
