>>278
じゃあクロームならこんな拡張使って
JavaScript Injector 　
https://chrome.google.com/webstore/detail/javascript-injector/djnjegpffahmfpjdlkciiecmeaebghlk

https://www.nicovideo.jp/watch/をurlに指定して
スクリプト
//Autoplay [HTML5]
window.addEventListener("load", function() {
setTimeout(function() {
function AutoPlayAttr() {
var vid = document.getElementsByTagName("video");
vid[0].setAttribute("autoplay", "");
};
AutoPlayAttr();
const target = document.querySelector("h1.VideoTitle");
const observer = new MutationObserver((mutations) => {
AutoPlayAttr();
});
const config = {
characterData: true
};
observer.observe(target, config);
}, 5);
});