FYI - on initial load of the demo site, in Chrome 80, on Windows 8.1:
The highlight slipped to the wrong characters.

(I'm still just trying to figure out how your stuff works, as I wanted to use your readability. Neat stuff, but I probably won't be able to figure out why this is off.)

Whenever the text begins to scroll,

1. The scrollbar adjusts the content width, which throws off the background highlighting positioning.

2. Scrolling scrolls the text, but the background highlighting doesn't seem to always scroll with it, often being off by several lines at a time.