Mysterious icon in the URL bar needs a tooltip
There's a mysterious icon on the right side of my URL bar. It looks like an open book. It has no tooltip. Clicking it while I have https://factorio.com/blog/post/fff-301 open changes the page background from dark to light and makes the fonts a bit bigger.
I think it might be for toggling readability mode a la Firefox, but it'd be nice if there was a tooltip to tell me what it actually does.