To minimize additional computation JPEGs often use a limited number of scans, for example 5. By default JPEG XL's progressive uses only two scans, 8x8 DC, and

To minimize additional computation JPEGs often use a limited number of scans, for example 5. By default JPEG XL's progressive uses only two scans, 8x8 DC, and the transforms in the second pass using a 256x256 tiles in encode-time chosen priority order. This choise allows JPEG XL to do only one round of DCTs even in the progressive case. The 8x8 DC is interpolated using cheaper methods. Because of the design choices, every JPEG XL image is guaranteed to be at least minimally progressive in the same manner, i.e., 8x8 DC first. Having a guarantee will make it more rewarding for system designers to focus on extracting some user-experience benefit from that feature.