A very nifty scroll to play script insipired by Vine.From http://jsbin.com/ocupor/1/edit

scrollPlay.js

var video = document.getElementById('video'),
    info = document.getElementById('info'),
    fraction = 0.8;

function checkScroll() {
  var x = video.offsetLeft,
      y = video.offsetTop,
      w = video.offsetWidth,
      h = video.offsetHeight,
      r = x + w, //right
      b = y + h, //bottom
      visibleX,
      visibleY,
      visible;

  if (window.pageXOffset >= r ||
      window.pageYOffset >= b ||
      window.pageXOffset + window.innerWidth < x ||
      window.pageYOffset + window.innerHeight < y
     ) {    
    
    info.innerHTML = '0%';
    return;
  }

  visibleX = Math.max(0, Math.min(w, window.pageXOffset + window.innerWidth - x, r - window.pageXOffset));
  visibleY = Math.max(0, Math.min(h, window.pageYOffset + window.innerHeight - y, b - window.pageYOffset));

  visible = visibleX * visibleY / (w * h);
  
  info.innerHTML = Math.round(visible * 100) + '%';
  
  if (visible > fraction) {
    video.play();
  } else {
    video.pause();
  }
  
}

checkScroll();
window.addEventListener('scroll', checkScroll, false);
window.addEventListener('resize', checkScroll, false);