IntersectionObserver

94.41

API that can be used to understand the visibility and position of DOM elements relative to a containing element or to the top-level viewport. The position is delivered asynchronously and is useful for understanding the visibility of elements and implementing pre-loading and deferred loading of DOM content.

Further reading:

1. Can I use page for: IntersectionObserver
2. MDN Web Docs - Intersection Observer
3. Polyfill
4. Google Developers article