gist-html-preview.js

// ==UserScript==
// @name         Gist HTML Live Preview
// @namespace    http://tampermonkey.net/
// @version      1.0
// @description  Display a button on GitHub Gist HTML files to preview the file using https://raw.githack.com to serve the page in developer mode.
// @author       Brian Mosher
// @match        https://gist.github.com/*
// @icon         data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==
// @grant        none
// ==/UserScript==
(function() {
    'use strict';
    let rawBtnLinks = document.querySelectorAll("div .file-actions > a");
    for (let link of rawBtnLinks)
    {
        let rawURL = link.href;
        let urlExtension = rawURL.split('.').pop();
        if ( urlExtension.toLowerCase() === 'html' || urlExtension.toLowerCase() === 'htm' )
        {
            let newPreviewLink = link.cloneNode();
            newPreviewLink.textContent = "Preview";
            newPreviewLink.href = newPreviewLink.href.replace("gist.github.com", "gist.githack.com");
            link.parentElement.appendChild(newPreviewLink);
        }
    }
})();