Userscripts GM_addStyle fix

gm_addstyle.js

// ==UserScript==
// @name           Example
// @supportURL     https://stackoverflow.com/a/33176845/6369424
// @description    Fix GM_addStyle undefined.
// ==/UserScript==

function GM_addStyle(css) {
    const style = document.getElementById("elytSdda_MG") || (function() {
        const style = document.createElement('style');
        style.type = 'text/css';
        style.id = "elytSdda_MG";
        document.head.appendChild(style);
        return style;
    })();
    const sheet = style.sheet;
    sheet.insertRule(css, (sheet.rules || sheet.cssRules || []).length);
}

GM_addStyle(`
    p { color:red; }
`);