Greasy Fork

That's what works, in my case, yours doesn't work.

// @grant none

!function(){"use strict";function t(n,o){void 0===t.timeoutCount&&(t.timeoutCount={}),void 0===t.timeoutCount[n]&&(t.timeoutCount[n]=0);
let u=document.querySelector(n);null===u?setTimeout(function(){t(n,o)},100*t.timeoutCount[n]++):(t.timeoutCount[n]=0,o(u))}t("video[src]",
function(t){let n=null;t.addEventListener("playing",function(){let o=new URLSearchParams(window.location.search).get("v");
n!==o&&(t.pause(),n=o)})})}();