<div class="triangle"></div>

<style>

  .triangle{

  border:10rem solid transparent;

  border-bottom-color:#c9ff21;

  width:0px;

  margin:auto;

  animation:1ms rotate;

  animation-timeline:scroll();

}

@keyframes rotate{

  to{

    rotate:y 5turn

  }

}</style>

body{

  height:150vh;

  display: flex;

  align-items:center

}