nolightfonts.js

// ==UserScript==
// @name        no light fonts
// @namespace   http://ikeran.org
// @description prevents sites from using font-weight below 400
// @include     *
// @version     1
// @grant       none
// ==/UserScript==

(function() {
  let min = 14; // px
  let max = 18; // px
  let hdrMax = 32; // px

  function fixFonts(elem) {
    if (!elem) return;
    let fs = window.getComputedStyle(elem, null).fontSize;
    let size = parseInt(fs);
    if (size < min) {
      size = min;
    }
    let t = elem.tagName.toLowerCase();
    if (t == "h1" || t == "h2") {
      if (size > hdrMax) {
        size = hdrMax;
      }
    } else if (size > max) {
      size = max;
    }
    size = "" + size + "px";
    elem.style.fontSize = size;

    let kids = elem.children;
    for (let i = 0; i < kids.length; i++) {
      fixFonts(kids[i]);
    }
  }

  var forbidFontWeight = function() {
    document.body.style.setProperty("font-weight", "400", "important");
  }
  var NAME = "dhasenan-font-weight";

  fixFonts(document.body);
  var cache = localStorage.getItem(NAME);
  if (cache) {
    if (cache.isSafe)
      return;
    else
      forbidFontWeight();
  }
  var elements = document.getElementsByTag('*');
  for (var i = 0 ; i < elements.length; i++) {
    if (window.getComputedStyle(elements[i]).getPropertyValue("font-weight") < 400) {
      localStorage.setItem(NAME, {isSafe: false});
      forbidFontWeight();
    }
  }
})();