We can't do content images yet due to print support and other load-event complexities (see parent task). But for non-printable ones we can start doing this.
The footer icons (Powered by MW, WMF project) might be a good place to start experimenting with the native browser feature for lazy loading of images.
- They are known to enjoy eager discovery by the browser's lookahead parser and thus get requested fairly early and thus compete for bandwidth with other low priority resources (such as async JS).
- They are not critical for the user experience.
- They are not critical for the above-the-fold rendering.
- They are hidden in the "print" mode stylesheet, which side-steps one of the two "known issues" with lazyload today.
- They are not part of the content, which means any JS code expecting images to have loaded at window.onload isn't affected (e.g. Mediaviewer), this side-steps the other known issue.