To mitigate the tracking problem, Firefox only defers loading of images when JS is enabled: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/im...

Chrome seemed to behave this way as well when I tested it a while ago. It's covered in the spec: https://html.spec.whatwg.org/multipage/urls-and-fetching.htm...

On the client side too. I use it to avoid loading below-the-fold images until it's necessary. For example, I don't load the thumbnails of related articles.

It makes a non-negligible impact on bandwidth usage and page load time.

Exactly this. It's a problem already solved by pagination - but I guess that's not cool and doesn't allow for such granular tracking.