Simple way to set html5 playback rate. Should probably be minified as a bookmarklet

setPlaybackRateFromPromptNaive

(function(){
  function setAllMediaPlaybackSpeed(speed) {
    [].slice.call(document.querySelectorAll('audio,video')).map( function(e, idx) {
      e.playbackRate = speed;
    });
  }
  var userRequestedRate = parseFloat(prompt("What playback rate would you like?"),10);
  if(!isNaN(userRequestedRate)) {
    setAllMediaPlaybackSpeed(userRequestedRate);
  }
})();