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 <head> 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.

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from CSS-Tricks https://www.smashingmagazine.com/2019/04/mutationobserver-api-guide/

Getting To Know The MutationObserver API
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