enlarge-arabic.js

// ==UserScript==
// @name        Enlarge Arabic
// @namespace   danlei
// @author      danlei
// @version     1.1.5
// @updateURL   https://gist.githubusercontent.com/danlei/17478a08b53ba9367e83111104bfa002/raw
// @installURL  https://gist.githubusercontent.com/danlei/17478a08b53ba9367e83111104bfa002/raw
// @include     /^https?://.*$/
// @exclude     https://twitter.com*
// @description Increase Arabic font size relatively (adapted from Arminius99's script)
// @grant GM_addStyle
// ==/UserScript==

// TODO: - what's wrong with twitter?
//       - test arabic in nested elements

GM_addStyle("span[lang~='ar'] {direction: rtl; font-size: 110% !important; }");

// cf. https://stackoverflow.com/a/58166435/271324
function addArabicSpan(textNode) {
    var div = document.createElement('div');
    textNode.parentNode.insertBefore(div, textNode);
    div.insertAdjacentHTML('afterend',
      textNode.data.replace(/((?=\s*[\u0600-\u06FF])[\u0600-\u06FF\s]+)/g,
                            "<span lang='ar'>$1</span>"));
    div.remove();
    textNode.remove();
}

[...document.querySelectorAll('*')].
  map(n => [...n.childNodes]).
  flat().
  filter(n => n.nodeType === 3).
  map(addArabicSpan);