Then from what you wrote ./charsheet/charsheet.html?p=${charCount} doesn’t update since theres no mecahnsim to revise static HTML inside an extension.
And if this internal HTML is running it’s own JS to fetch offsite JSON to build the page with, you are over complicated by trying to use iFrames. You can just dynamically write instead, to the HTML you already have, not an embedded page
Either way: you can do what you are trying to do, but you can’t do it the way you are trying to do it