I had a Chrome extension installed about 6 months ago that did this (and more). Click a button and an alert message would pop up, telling you which kanji on the page belonged to which WK level, and which were not on WK. Then I took a break and the extension disappeared from browser. Was it all a dream?
It’s probably looki’s highlighter script here: /t/Userscript-Highlight-Kanji-based-on-WK-level-in-websites/4998/1
So it wasn’t looki’s one, but hot damn! That one is much better. Thanks!