HTML and displaying of <img/> element

The idea of using a progressive JPEG in a website has less to do with saving bandwidth and more to do with showing something to the end user earlier. The browser still downloads the full image no matter what, but progressive JPEGs allow the user to see something happening earlier.

The W3 page on JPEG says this:

Progressive JPEG is a means of reordering the information so that, after only a small part has been downloaded, a hazy view of the entire image is presented rather than a crisp view of just a small part.

Because the inclusion of an IMG tag generally means that you want the browser to download that asset, it's not going to abort it partway through simply because it's determined it's downloaded "enough". The browser also doesn't know what you intend to do with the image later - maybe you'll zoom it later on, or maybe it'll need to show it later at a different size. Partially downloading once means that the browser might have to download again later.

And the Wikipedia article on JPEG points out:

However, progressive JPEGs are not as widely supported,[citation needed] and even some software which does support them (such as versions of Internet Explorer before Windows 7)[12] only displays the image after it has been completely downloaded.

So even if some browsers did abort connections for these after having downloaded "enough", not enough browsers provide support for me to consider even using them in the first place.