Many years ago (when the Internet was slow), I saw websites that showed JPG images while the browser downloading them.
I'm not speaking about two versions of images. The browser first downloads low-quality and then downloads the high-quality versions. I am speaking about one file such that while the browser downloads it, it shows the low quality version of the image (from the first bytes) until the image is ready.
Do you know how do it and what is the name of that technique?