You can put some simple CSS and still leave the simple HTML available to be styled by the browser's reader mode.

You're never going to please everyone. I've seen HNers complain about the opposite, that they have a large monitor for a reason and the web designer has wronged them by adding margins.

Anyways, I think we're all big boys and girls who can figure out how to use reader mode or resize a window in 2020. That's the whole beauty of the web browser.

It should have a sane default. The current default presentation is not sane.

Your user agent should have a sane default. The idea that every text document on the web should ship their own default is insane.