On the other hand, it also doesn't constantly change the DOM, moving you across the page because images above what you're reading are getting swapped in and now you're looking at the paragraph above the one you were reading.

and again.

oh, and again.

and-

The progressive mode in Jpeg and JPEG XL is quite different, because the quality is so much better your perception of it changes. Where Progressive Jpeg are literally useless before it finish loading, JPEG XL provides decent quality.

true progressive mode actually gives you something usable before it's fully loaded, to the point where "not fully loading" can be entirely acceptable. If all images load "first iteration" first, you have a stable DOM that won't need to reflow in any way as each image goes through the rest of its progressive layers. And having a nice setting that says "only load up to X on slow networks" and "require image click to load past first iteration" would be godsend, even on gigabit connections. If I could bring the size and load time for modern webpages back down to "maybe a few 10s of kb" and "immediate" rather than "2.9MB and a second or two", I'd happily turn those options on.

> Progressive mode lets you ignore it before it's fully loaded.

Huh? It's obviously easier to ignore the spinner than to ignore a low-res image. You've seen the spinner before.

How so? Progressive JPEGs load in multiple passes. The first pass trades fidelity for speed, successive passes add quality over time. Seems pretty much in line with what PWA is all about.

It entirely depends on what progressive steps you define / where you stop loading.