It is a hint, inasmuch as the behaviour depends on the lazy load root margin which is implementation-defined, with a handful of suggested factors to take into a

It is a hint, inasmuch as the behaviour depends on the lazy load root margin which is implementation-defined, with a handful of suggested factors to take into account in choosing the defaults. Disabling lazy loading is done by setting the lazy load root margin infinitely large. Relevant spec: https://html.spec.whatwg.org/multipage/urls-and-fetching.html#lazy-load-root-margin https://html.spec.whatwg.org/multipage/urls-and-fetching.htm.... In Firefox, you can control this by setting the dom.image-lazy-loading.root-margin.{top,left,right,bottom} prefs in about:config.