Script for Tampermonkey that adds 'view' to the actions menu beside 'row', 'blame' and 'history' buttons and allow you to preview rendered .html pages.

github-html-preview-monkey.js

// ==UserScript==
// @name         Github html preview
// @author       https://github.com/vaniakosmos
// @version      1.0
// @description  Shortcut for htmlpreview.github.io
// @match        https://github.com/**/*.html
// @grant        none
// @require http://code.jquery.com/jquery-3.2.1.min.js
// ==/UserScript==

$(document).ready(function() {
    var viewBtn = $('<a>')
        .attr('href', 'http://htmlpreview.github.io/?' + window.location.href)
        .attr('class', 'btn btn-sm BtnGroup-item')
        .html('Html preview');
    $('.file-actions').find('.BtnGroup').append(viewBtn);
});