offscreen-images suggests lazy loading any images outside of the initial viewport (plus a little leeway on all sides).

After discussion in ampproject/amphtml#25682 and in the latest eng meeting, we've decided to bump that up to three viewports. It's still a little arbitrary, but the impact is minimal, it lines up with the leniency of other lazy loading approaches (like the very large buffer in native lazy loading), and it's arguably a better user experience.

whatwg discussion on this issue: whatwg/html#5408