Ugh, the font doesn't scale when zooming, but the pictures do. The font does scale though when changing the container size. The font is massive on a monitor, terrible UX.
The author used viewport-relative units for body text and (guessing) only tested on a laptop screen.
This fixes it:
[data-predefined-style="true"] main { font-size: 20px; }
Also: It sucks that bookmarklets have fallen out of fashion. I just spent a few minutes trying to find a working bookmarklet that changes the font size and failed.