You're free to narrow your browser window — that's what I used to do, before all web sites decided that they know better than I do what width their text should be.

Agreed.

body{max-width:640px;margin:auto} The extra 33 bytes won't slow things down (unless you somehow hit the next ~1kb packet boundary)

line-height 1.5; would also make it more readable.

640px would be a problem — that's approximately 15em on my screen. I don't think that unit is supposed to be about hardware pixels, but...

Well, that's how it seems to be implemented anyway. Ems usually do better.

On hidpi screens they're emulated pixels, not hardware pixels (pretty much for this reason).

That's the idea. XFCE doesn't deal well with hidpi.

It, um, only fills two-thirds.

That's a compromise. Too many sites don't deal well with thin browsers, but I need space for my terminals. This width usually works, although I sometimes have to do a bit of horizontal scrolling to get the article fully visible. (As opposed to the sadly inevitable sidebars.)

Margins are good, though.

My browser fills however much of my screen dwm tells it to fill.