MutationObserver watches the DOM, specifically the places you tell it to, like:
…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
<head> of a Pen and detected newly-injected processed Sass code, so you could use CodePen to see Sass and CSS side by side:
from CSS-Tricks https://www.smashingmagazine.com/2019/04/mutationobserver-api-guide/