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    <li><a href="#">TOP</a></li>
    <li><a href="#">PRODUCT</a></li>
    <li><a href="#">ABOUT</a></li>
    <li><a href="#">NEWS</a></li>
    <li><a href="#">CONTACT</a></li>


                nav ul {
  display: flex;
  justify-content:  space-between;
  /* flexで横並び。お好みで変更してね*/

nav ul li{
  list-style: none;
  /* 箇条書きの「・」を無くす*/

/* hoverでボーダーボトム出現 */
nav ul li a{
  font-size: 10px;
  color: black;
  padding-bottom: 8px;
  /* 箇条書きの「・」を無くす*/
  text-decoration: none;
  /* リンクの下線を消します。*/
  border-bottom: 2px solid transparent;
    /* transparentで常に透明にする*/
  transition-duration: 1s;
   /* ボーダーが出現するまでの時間です。

nav ul li a:hover{
  border-color: black;
   /* :hoverすると透明だったボーダーが黒くなります*/
   /* 要するに、最初から存在するけど、出現するように見えるんです*/