<ol class="cp_clist">
  <li>css</li>
  <li>html</li>
  <li>copy</li>
  <li>easy</li>
</ol>

ol.cp_clist {
  padding: 0;
  counter-reset:cp_clist; /*数字をリセット*/
  list-style-type: none;
}
ol.cp_clist li {
  position: relative;
  padding: 0 5px;
  margin: 0 0 5px 40px;
  min-height: 28px;
  background: #FFAB91;
  color: #333;
  transition: all .3s ease-out;
}
ol.cp_clist li::before {
  position: absolute;
  top: 0;
  left: -10px;
  display: flex;
  justify-content: center;
  align-items: center;
  /* カウントさせる */
  counter-increment: cp_clist;
  content: counter(cp_clist);
  /*装飾*/
  color: #fff;
  background: #FF5722;
  height: 28px;
  width: 28px;
  /*位置の調整*/
  padding: 0;
  margin: 0px 0 0 -30px;
}
ol.cp_clist li::after {
  content: '';
  position: absolute;
  top: 10px;
  left: -17px;
  background: #FF5722;
  width: 10px;
  height: 10px;
  clip-path: polygon(0 0, 0% 100%, 50% 50%);
  transition: all .3s ease-out;
}
/*hover時の設定*/
ol.cp_clist li:hover {
  background: #FFCCBC;
}
ol.cp_clist li:hover::after {
  left: -6px;
}