Most graphical WWW browsers can display JPEG files. Particularly nice ones are able to display a JPEG file as it loads across a slow network connection.
Early passes contain coarse-grain image information, and later passes contain finer detail.
This means that a complete, but low-quality image can be displayed very soon. As more data arrives, the image quality increases, as though coming into finer focus. If you are going to put progressive-display JPEGs in WWW pages, you may wish to warn readers of this incompatibility. You can also use it to convert progressive-display JPEGs into sequential mode, so that older browsers and viewers can display them. Now drag JPEG files or folders onto the Progressify icon.