code: function addAnimation(alpha, trail, i, lines) {
      var name = ['opacity', trail, ~~(alpha*100), i, lines].join('-')
        , start = 0.01 + i/lines*100
        , z = Math.max(1 - (1-alpha) / trail * (100-start), alpha)
        , prefix = useCssAnimations.substring(0, useCssAnimations.indexOf('Animation')).toLowerCase()
        , pre = prefix && '-'+prefix+'-' || ''
  
      if (!animations[name]) {
        sheet.insertRule(
          '@' + pre + 'keyframes ' + name + '{' +
          '0%{opacity:' + z + '}' +
          start + '%{opacity:' + alpha + '}' +
          (start+0.01) + '%{opacity:1}' +
          (start+trail) % 100 + '%{opacity:' + alpha + '}' +
          '100%{opacity:' + z + '}' +
          '}', sheet.cssRules.length)
  
        animations[name] = 1
      }
      return name
    }
docs: Creates an opacity keyframe animation rule and returns its name.
  Since most mobile Webkits have timing issues with animation-delay,
  we create separate rules for each line/segment.