chrome F12 debugger
Inspect
Ctrl+Shift+C then click the element. Or right click the element and inspect.
CSS editor
In the bottom of elements tab, it has a style tab that shows the CSS rules applied.
It is just so helpful QwQ.

Pitfalls
- JS caching: chrome will cache the
javascriptcode by default, useCtrl + F5FULL refresh to make sure the code is up to date.