Comment by DamonHD

Browsers already have an early scanner to look ahead for things that it may need to load soon, such as images, and piles of heuristics. Those heuristics are hard in part because many HTML authors don't bother marking up their image dimensions. The lazy attribute helps avoid loading images that the author can be fairly sure will not be in the initial viewport, so is an optimisation hint to override some of those heuristics. So it saves some bandwidth and helps ensure that things above the fold are not fighting things below in the initial viewport construction. So we're about two levels of optimisation in here, but browsers do a reasonable job when fed good img tags anyway.