> Why? These are all easily customizable by the user.
monospace and sans-serif are customizable. Helvetica isn't. Regarding font size, not that long ago there was a post here from someone who set "font-size: 1.4em" in their CSS, because they couldn't be bothered to set up HiDPI scaling on their system. Mind you this CSS rule doesn't set the font size to anything specific. It tells the browser "whatever size you wanted to use for this text, make it 1.4 times bigger". So if user set say 14px as their preferred font size, it would be 19.6 px instead. That's just asshole design.