<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;
  margin: 0 0 0 25px;
}
ol.cp_clist li::before {
  position: absolute;
  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*/
  background: #4CAF50;
  color:#fff;
  height: 20px;
  width: 20px;
  clip-path: inset(0px round 10px 1px);
  /*位置の調整*/
  padding: 0;
  margin: 2px 0 0 -25px;
}