When JPEG images are used by a web page, they are typically rendered top-down ... but they can also be rendered using a mode called **progressive JPEG**, where the image starts out **full-size**, but *blurry*, and then gets sharper with successive passes, until it's fully loaded. **Progressive** loading requires the image have been saved that way.

Why don't more web sites use progressive JPEG? What are the drawbacks? Is it simply a lack of tool support, or are these files somehow inferior to traditional top-down rendered JPEG images?