On Container Queries, Responsive Images, and JPEG-XL

A progressive image format doesn't really fully solve the problem, it's only a potential bandwidth improvement. In both the case of a progressive and non-progressive image, you potentially prefetch the wrong asset - either a too low resolution image, or too few bytes of the progressive image, and you are stuck either waiting or displaying a too low resolution version of the image. The only advantage of the progressive format is that the prefetch bytes aren't wasted - as a rough estimate, if you grabbed the 1x image instead of 2x, you ended up using 125% as much bandwidth as intended. A nice improvement but also much smaller than just using a better image format in general. Likewise, it doesn't remove the need for container queries, you'd still at least want a byte offset for the complete base layer.