scroll check : https://jsfiddle.net/dohoons/xg29zo4m/

scroll.js

// scroll check
(function() {
  var $window = $(window);
  var $target = $('.scroll-check');

  function scrollHandle() {
    var winHeight = $window.height();
    var scrollTop = $window.scrollTop();

    $target.each(function() {
      var $this = $(this);
      var ratio = 1 - (Number($this.data('ratio')) || 80) / 100;
      var isViewportIn = scrollTop > ($this.offset().top - winHeight) + (winHeight * ratio);
      $this.toggleClass('in', isViewportIn);
    });
  }

  $target.addClass('ready');
  $window.on('resize scroll', scrollHandle);
  scrollHandle();
})();