mdn-links.js

// ==UserScript==
// @name         MDN anchor links
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @match        https://developer.mozilla.org/*
// @grant        none
// ==/UserScript==

(function() {
  'use strict';

  document.head.insertAdjacentHTML('beforeEnd', `
    <style>
      *:hover > .silly-anchor-links {
        content: "";
        width: 28px;
        height: 28px;
        position: absolute;
        background: gray;
        box-shadow: inset 0 0 0 5px red, inset 0 0 0 10px white;
        border-radius: 50%;
      }
    </style>
  `);

  document.querySelectorAll('[id]').forEach(el => el.insertAdjacentHTML('afterBegin', `<a class="silly-anchor-links" href="#${el.id}"></a>`));
})();