This can be solved by installing ttf-mscorefonts-installer and accepting the proprietary license.
I have looked at HN's CSS, and it seems okay (has a fallback to a generic font family), but the problem I think is that the default Linux font for sans-serif is too tiny when the size is set to 10 pt. Verdana, ..., look fine at that size.
I would recommend that the site's CSS be changed in some way (I don't quite know how exactly) to fix the issue with small fonts on Linux.
P.S. Happens on Debian/Ubuntu/Linux Mint with Chrome and Firefox.
// ==UserScript==
// @name hn_resize_subtitle
// @namespace com.kludgecode.hn.demo
// @include https://news.ycombinator.com*
// @version 1
// @grant none
// ==/UserScript==
window.$hn_subtext_text_target = document.getElementsByClassName("subtext");
for (var i = 0; i < $hn_subtext_text_target.length; i++)
{$hn_subtext_text_target[i].style.fontSize = 12;};
They all should have additional settings:
1. minimum font size
2. maximum font size
to solve this kind of problems once and for all. I am actually very puzzled why nobody ever thought of that.