A Few Functional Uses for Intersection Observer to Know When an Element is in View

MutationObserver watches the DOM, specifically the places you tell it to, like:
document.querySelector(#watch-this);
…and it can tell you (trigger a callback) when stuff happens — like when a child is added, removed, changed, or a number of other things.
I used it just the other day to watch the of a Pen and detected newly-injected processed Sass code, so you could use CodePen to see Sass and CSS side by side:
See the Pen
Sassmeister clone by Chris Coyier (@chriscoyier)
on CodePen.
Direct Link ?