yt_replay.js

(function() { 'use strict';
  var currentTimeEl = document.querySelector('.ytp-time-current'),
      endTimeEl = document.querySelector('.ytp-time-duration'),
      replayBtnEl = document.querySelector('.ytp-play-button');

  setTimeout(function replay() {
    if (
      (currentTimeEl.textContent == endTimeEl.textContent) 
        &&
      (replayBtnEl.getAttribute('title') == 'Replay')
    ) {
      replayBtnEl.click();
    }

    setTimeout(replay, 1000);
  }, 1000)
})()