This userscript adds a button for switching plain/rich for a Markdown blob on GitHub.

switch-plain-md-github.user.js

// ==UserScript==
// @name         Switching Plain for Markdown on GitHub
// @namespace    hexium310
// @version      0.3
// @description  This userscript adds a button for switching plain/rich for a Markdown blob on GitHub.
// @author       Hexin
// @match        https://*.github.com/*/*
// @downloadURL  https://gist.github.com/hexium310/76eda31e03f7bd09227133a26364bbe8/raw/switch-plain-md-github.user.js
// @updateURL    https://gist.github.com/hexium310/76eda31e03f7bd09227133a26364bbe8/raw/switch-plain-md-github.user.js
// @grant        none
// ==/UserScript==

(() => {
  'use strict';

  const toCamelCase = (str) => str.replace(/^./, (v) => v.toUpperCase());

  const addButton = () => {
    const rawButton = document.getElementById('raw-url');
    if (!rawButton) {
      return;
    }
    const url = new URL(location.href);
    const newButton = rawButton.cloneNode(true);

    const isPlain = url.searchParams.has('plain');
    const kind = isPlain ? 'rich' : 'plain';

    if (isPlain) {
      url.searchParams.delete('plain');
    } else {
      url.searchParams.set('plain', 1);
    }

    newButton.id = `${ kind }-url`;
    newButton.textContent = ` ${ toCamelCase(kind) } `;
    newButton.href = url;

    rawButton.parentNode.insertBefore(newButton, rawButton);
  };

  addButton();
  document.addEventListener('pjax:end', addButton);
})();