// ==UserScript==
// @name        Imgur gifv as webm
// @namespace   http://tvkdevelopment.com
// @include     http*://i.imgur.com/*.gifv
// @version     1
// @grant       none
// ==/UserScript==

document.body.style.textAlign = 'center';
document.body.innerHTML = '<video controls loop autoplay style="position:relative;top:50%;transform:translateY(-50%);">\
  <source src="' + window.location.href.replace(".gifv", ".webm") + '" type="video/webm">\
</video>';