This Greasemonkey script transforms all outdated LJ placeholders for YouTube videos to valid HTML5 objects

LJ_YouTube_HTML5.user.js

// ==UserScript==
// @name        LJ HTML5
// @namespace   lj-youtube-html5
// @include     https://*.livejournal.com/*
// @include     http://*.livejournal.com/*
// @version     1.1
// @grant       none
// ==/UserScript==

function replace(frame, regexp){
  var match = frame.src.match(regexp);
  var youtubeid = match ? match[1] : null;
  if (youtubeid) {
    frame.src = "//www.youtube.com/embed/".concat(youtubeid, "?rel=0");
  }
}

var frames = document.getElementsByClassName("lj_embedcontent");
for (var i = 0; i < frames.length; i++) {
  var re = new RegExp("^http://l.lj-toys.com/.*vid=([^&]+)(&|$)");
  replace(frames[i], re);
}

var frames = document.getElementsByTagName('iframe');
for (var i = 0; i < frames.length; i++) {
  var re = new RegExp("^http://www.youtube.com/embed/([^&]+)\\?wmode=opaque(&|$)");
  replace(frames[i], re);
}