fadeIn vanilla JS

fade_in.js

function fadeIn(el) {
  var opacity = 0

  el.style.opacity = 0
  el.style.filter = ''

  var last = +new Date
  var tick = function() {
    opacity += (new Date - last) / 400
    el.style.opacity = opacity
    el.style.filter = 'alpha(opacity=' + (100 * opacity)|0 + ')'

    last = +new Date

    if (opacity < 1)
      (window.requestAnimationFrame && requestAnimationFrame(tick)) || setTimeout(tick, 16)
  }

  tick()
}

fadeIn(el)