Firefox reader view is made for this issue. Adjust the content width slider a bit and it's great.

Ironically, making my Firefox window smaller - about 830 pixels wide - made the text content wider, taking up the full width.

Making it wider than about a thousand pixels shrinks the text column down to 400 pixels.

A weird design choice.

fwiw, I like to set max-width of columns of text to about `64ch`. In smaller view ports I may go down to about 45ch. But I feel the range of 50-65ch is ideal for reading. Setting to 20% or 20 vw is a bit dangerous for this precise issue.

It was unpleasant on a 27" 1440p monitor as well. I think the content is just too narrow in general.

The CSS of that website is weird in general, seems to be coming from whatever the kids use today that works like Dreamweaver. The width is so small because the layout is a grid for some reason, although it's just two columns. That same column has a "min-width: 0" and seems like that's happening in more places too, properties set to their defaults? Looks really strange overall.

This is why good browsers have a reader mode. Click a button and you can make it easily readable for you if you have challenges reading. I don't know why you would resize the window. Just use reader mode.