The Tampermonkey userscript to download video and audio from Youtube, Twitter, Vimeo, Facebook, Dailymotion, 1tv, VK, youku, bilibili and 5000 more sites for free. Fast and easy to use.

Video_Audio_Downloader.js

// ==UserScript==
// @name         Distill Video & Audio Downloader from 5000+ sites including Youtube, Support 1080P, 2K, 4k & 8K
// @namespace   https://distillvideo.com/
// @version     2.1.1
// @date        2018-06-17
// @description Browser extension to download video and audio from Youtube, Twitter, Vimeo, Facebook, Dailymotion, 1tv, VK, youku, bilibili and 5000 more sites for free. Fast and easy to use.
// @author      DistillVideo.com
// @copyright   2018, DistillVideo.com
// @homepage    https://distillvideo.com/page/extensions
// @downloadURL https://distillvideo.com/js/ditillvideo.user.js
// @compatible chrome
// @compatible firefox
// @compatible opera
// @compatible safari
// @license GNU GPL v3.0 or later. http://www.gnu.org/copyleft/gpl.html
// @match          *://*.youtube.com/*
// ==/UserScript==

(function() {
    'use strict';

    if (document.getElementById("polymer-app") || document.getElementById("masthead") || window.Polymer) {
    setInterval(function() {
        if (window.location.href.indexOf("watch?v=") < 0) {
            return false;
        }
        if (document.getElementById("count") && document.getElementById("distillvideo") === null) {
            Addytpolymer();
        }
    }, 100);
} else {
    setInterval(function() {
        if (window.location.href.indexOf("watch?v=") < 0) {
            return false;
        }
        if (document.getElementById("watch7-subscription-container") && document.getElementById("distillvideo") === null) {
            AddhtmlDV();
        }
    }, 100);
}

function AddhtmlDV() {
    if (document.getElementById("watch7-subscription-container")) {
        var wrap = document.getElementById('watch7-subscription-container');
        var button = "<div id='distillvideo' style='display: inline-block; margin-left: 10px; vertical-align: middle;'>";
        button += "<a href=\"https://distillvideo.com/?url=https://www.youtube.com/watch?v=" + disvidyouvid(document.URL) + "\" title=\"Download this video\" target=\"_blank\"" +
            "style=\"display: inline-block; font-size: inherit; height: 22px; border: 1px solid rgb(0, 183, 90); border-radius: 3px; padding-left: 28px; cursor: pointer; vertical-align: middle; position: relative; line-height: 22px; text-decoration: none; z-index: 1; color: rgb(255, 255, 255);\">";
        button += "<i style=\"position: absolute; display: inline-block; left: 6px; top: 3px; background-image: url(data:image/svg+xml;base64,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); background-size: 12px; background-repeat: no-repeat; background-position: center center; width: 16px; height: 16px;\"></i>";
        button += "<span style=\"padding-right: 12px;\">Download</span></a></div>";
        var style = "<style>#distillvideo button::-moz-focus-inner{padding:0;margin:0}#distillvideo a{background-color:#15388c}#distillvideo a:hover{background-color:#E91E63}#distillvideo a:active{background-color:rgb(0, 151, 74)}</style>";
        var tmp = wrap.innerHTML;
        wrap.innerHTML = tmp + button + style;
    }
}

function Addytpolymer() {
    var buttonDiv = document.createElement("span");
    buttonDiv.style.width = "100%";
    buttonDiv.id = "distillvideo";
    var addButton = document.createElement("a");
    addButton.appendChild(document.createTextNode("Download video"));
    addButton.style.width = "100%";
    addButton.style.backgroundColor = "#15388c";
    addButton.style.color = "white";
    addButton.style.textAlign = "center";
    addButton.style.padding = "5px 10px";
    addButton.style.margin = "0px 10px";
    addButton.style.fontSize = "14px";
    addButton.style.border = "0";
    addButton.style.cursor = "pointer";
    addButton.style.borderRadius = "2px";
    addButton.style.fontFamily = "Roboto, Arial, sans-serif";
    addButton.style.textDecoration = "none";
    addButton.href = "https://distillvideo.com/?url=https://www.youtube.com/watch?v=" + disvidyouvid(window.location.href);
    addButton.target = "_blank";
    buttonDiv.appendChild(addButton);
    var targetElement = document.querySelectorAll("[id='count']");
    for (var i = 0; i < targetElement.length; i++) {
        if (targetElement[i].className.indexOf("ytd-video-primary-info-renderer") > -1) {
            targetElement[i].appendChild(buttonDiv);
        }
    }
}

function disvidyouvid(url) {
    var p = /((http|https)\:\/\/)?(?:[0-9A-Z-]+\.)?(?:youtu\.be\/|youtube(?:-nocookie)?\.com\S*[^\w\s-])([\w-]{11})(?=[^\w-]|$)(?![?=&+%\w.-]*(?:['"][^<>]*>|<\/a>))[?=&+%\w.-]*/ig;
    return (url.match(p)) ? RegExp.$3 : false;
}

})();

Button didn't appear, Site didn't work.

Doesn't work for any site just tried it.