Fosdem Chromecastplayer.user.js

// ==UserScript==
// @name       Fosdem Chromecastplayer
// @version    1.0
// @description  Adds link to cast to Chromecast using Chromecast.link
// @match      live.fosdem.org/watch*
// @copyright  2021, Olaf
// ==/UserScript==

function _x(STR_XPATH) {
    var xresult = document.evaluate(STR_XPATH, document, null, XPathResult.ANY_TYPE, null);
    var xnodes = [];
    var xres;
    while (xres = xresult.iterateNext()) {
        xnodes.push(xres);
    }

    return xnodes;
}

(function() {
    'use strict';
var child = _x('/html/body/div[3]/p[2]/a[1]')
var blah = document.createElement('a')
blah.setAttribute('href', 'https://chromecast.link/#content=' + encodeURI(child[0].href))
blah.innerHTML = '<br/>Cast!'
child[0].append(blah)
})();