Doesn't work well inside an iframe or an element with overflow

[dizel3d]

Doesn't work well:

1. inside an iframe that isn't visible in top window viewport.
2. inside an element with overflow, e.g. inside div { width: 0; height: 0; overflow: hidden; }.

In the cases above LazyLoad shouldn't trigger loading, but it does.

Using IntersectionObserver would be a solution.

[dizel3d]

One more advice about IntersectionObserver #123