Interesting research from Aaron Peters on <img loading="lazy" ... >:
On my 13 inch macbook, with Dock positioned on the left, the viewport height in Chrome is 786 pixels so images with
loading="lazy"that are more than 4x the viewport down the page are eagerly fetched by Chrome on page load.In my opinion, that is waaaaay too eager. Why not use a lower threshold value like 1000 pixels? Or even better: base the threshold value on the actual viewport height.
My guess is they chose not to over-engineer the feature by default and will improve it over time. By choosing a fairly high threshold, they ran a lower risk of it annoying users with layout shifts on pages with images that don’t use width/height attributes.
I think this unmerged Pull Request is the closest thing we have to a spec and it uses language like “scrolled into the viewport” which suggests no threshold at all.