Nice! I built something similar for https://histre.com/ and made it even robust that the highlight will stay on even if the rest of the page changes. For example, if you highlight a sentence in Hacker News comments, the position of the comment changes all the time and histre highlights the correct sentence even with that change.
I open sourced it here, if that's helpful: https://gitlab.com/histre/browser-extension/-/blob/master/hi...