<div class="cp_ipselect06">
  <select class="cp_sl06" required>
    <option value="" hidden disabled selected></option>
    <option value="1">cat</option>
    <option value="2">dog</option>
    <option value="3">rabbit</option>
    <option value="4">squirrel</option>
  </select>
  <span class="cp_sl06_selectbar"></span>
  <label class="cp_sl06_selectlabel">Choose</label>
</div>

.cp_ipselect06 {
  position: relative;
  display: flex;
  justify-content: center;
  flex-direction: column;
  margin: 2em auto;
}
.cp_ipselect06 .cp_sl06 {
  position: relative;
  width: 100%;
  height: 48px;
  cursor: pointer;
  text-overflow: ellipsis;/*テキストがオーバーしたら'...'で省略*/
  z-index: 1;
  /* 標準のスタイルを無効にしborder-bottomを設定 */
  border: none;
  appearance: none;
  outline: none;
  background: transparent;
  border-radius: 0;
  border-bottom: 1px solid #666666;
}
/*プルダウンの三角を設定*/
.cp_ipselect06::after {
  position: absolute;
  right: 15px;
  content: '';
  width: 16px;
  height: 8px;
  background: #666666;
  clip-path: polygon(0 0, 100% 0, 50% 100%);
}
.cp_sl06_selectlabel {
  position: absolute;
  left: 0;
  top: 10px;
  color: #666666;
  transition: 0.2s ease all;
}
/*選択した際の動き:「Choose」の文字*/
.cp_sl06:focus ~ .cp_sl06_selectlabel,
.cp_sl06:valid ~ .cp_sl06_selectlabel {
  color: #da3c41;
  top: -20px;
  transition: 0.2s ease all;
  font-size: 80%;
}
/*選択した際の動き:セレクト下のライン*/
.cp_sl06_selectbar {
  position: relative;
  display: block;
  width: 100%;
}
.cp_sl06_selectbar:before,
.cp_sl06_selectbar:after {
  bottom: 1px;
  content: '';
  width: 0;
  height: 2px;
  position: absolute;
  background: #da3c41;
  transition: 0.2s ease all;
}
.cp_sl06_selectbar:before {
  left: 50%;
}
.cp_sl06_selectbar:after {
  right: 50%;
}
/*focus時の設定*/
.cp_ipselect06 .cp_sl06:focus {
  border-bottom: 1px solid transparent;
}
.cp_sl06:focus ~ .cp_sl06_selectbar:before,
.cp_sl06:focus ~ .cp_sl06_selectbar:after {
  width: 50%;
}