Progressive decoding isn't a very useful video feature because you need to decode the whole frame before you decode the next frame for inter-frame coding methods anyway.
It is, however, a very useful image format feature, giving you most information from the spinner/sticker/emoji/sprite, and refinig the already playing loop as it loads. That's why animated jxl is a bad video format — it's not a video format, it is a separate kind of seemingly weird thing.
