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About the <head>

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CSS

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CSS Base

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <head>
  <meta name="viewport" content="width=device-width,initial-scale=1.0">
  <!--==============レイアウトを制御する独自のCSSを読み込み===============-->
  <link rel="stylesheet" type="text/css" href="https://assets.codepen.io/6329135/reset.css">
</head>

<body>
  <div class="btn-position">
    <a href="#" class="btn bgleft"><span>背景が流れる（左から右）</span></a>
    <a href="#" class="btn bgright"><span>背景が流れる（右から左）</span></a>
    <a href="#" class="btn bgtop"><span>背景が流れる（上から下）</span></a>
    <a href="#" class="btn bgbottom"><span>背景が流れる（下から上）</span></a>
    <a href="#" class="btn bgcenterx"><span>背景が流れる（中央から横全体）</span></a>
    <a href="#" class="btn bgcentery"><span>背景が流れる（中央から縦全体）</span></a>
    <a href="#" class="btn bgcenterout"><span>背景が流れる（中央から外）</span></a>
    <a href="#" class="btn bgskew"><span>背景が流れる（斜め）</span></a>
  </div>
</body>
!

CSS 

              
                @charset "utf-8";

/*== 背景共通設定 */
body {
  width: 100%;
  height: 100vh;
  background-repeat: no-repeat;
  background-position: center;
  background-size: cover;
  background-image: url("https://assets.codepen.io/6329135/img_06.jpg");
  object-fit: cover;
}

/*== ボタン共通設定 */
.btn {
  /*アニメーションの起点とするためrelativeを指定*/
  position: relative;
  overflow: hidden;
  /*ボタンの形状*/
  text-decoration: none;
  border: 1px solid rgba(255, 255, 255, 0.18);
  display: inline-block;
  padding: 20px 0;
  margin: 10px 0;
  text-align: center;
  outline: none;
  transition: ease 0.2s; /*アニメーションの指定*/
  width: clamp(240px, 50vw, 500px); /*ボタンの横幅を全て統一*/

  background-color: rgba(255, 255, 255, 0.3);
  backdrop-filter: blur(6.5px);
  -webkit-backdrop-filter: blur(6.5px);

  box-shadow: 0px 6px 15px 0px rgba(142, 142, 142, 0.19);
  -webkit-box-shadow: 0px 6px 15px 0px rgba(142, 142, 142, 0.19);

  border-radius: 12px;
  -webkit-border-radius: 12px;
  color: rgba(255, 255, 255, 0.898);
}

/*ボタンの配置*/
.btn-position {
  display: flex;
  flex-direction: column;
  align-items: center;
}

/*ボタン内spanの形状*/
.btn span {
  position: relative;
  z-index: 3; /*z-indexの数値をあげて文字を背景よりも手前に表示*/
  color: #333;

  font-size: clamp(0.8rem, 2vw, 1rem);
}

.btn:hover span {
  color: #fff;
}

/* ==== 背景が流れる（左から右） ==== */
.bgleft:before {
  content: "";
  /*絶対配置で位置を指定*/
  position: absolute;
  top: 0;
  left: 0;
  z-index: 2;
  /*色や形状*/
  background: rgba(90, 150, 255, 0.5);
  width: 100%;
  height: 100%;
  /*アニメーション*/
  transition: transform 0.6s cubic-bezier(0.8, 0, 0.2, 1) 0s;
  transform: scale(0, 1);
  transform-origin: right top;
}

/*hoverした際の形状*/
.bgleft:hover:before {
  transform-origin: left top;
  transform: scale(1, 1);
}

/* ==== 背景が流れる（右から左） ==== */
.bgright:before {
  content: "";
  /*絶対配置で位置を指定*/
  position: absolute;
  top: 0;
  left: 0;
  z-index: 2;
  /*色や形状*/
  background: rgba(90, 150, 255, 0.5);
  width: 100%;
  height: 100%;
  /*アニメーション*/
  transition: transform 0.6s cubic-bezier(0.8, 0, 0.2, 1) 0s;
  transform: scale(0, 1);
  transform-origin: left top;
}

/*hoverした際の形状*/
.bgright:hover:before {
  transform-origin: right top;
  transform: scale(1, 1);
}

/* ==== 背景が流れる（上から下） ==== */
.bgtop:before {
  content: "";
  /*絶対配置で位置を指定*/
  position: absolute;
  top: 0;
  left: 0;
  z-index: 2;
  /*色や形状*/
  background: rgba(90, 150, 255, 0.5);
  width: 100%;
  height: 0;
  /*アニメーション*/
  transition: 0.3s cubic-bezier(0.8, 0, 0.2, 1) 0s;
}

/*hoverした際の形状*/
.bgtop:hover:before {
  height: 100%;
  background-color: rgba(90, 150, 255, 0.5);*
}

/* ==== 背景が流れる（下から上） ==== */
.bgbottom:before {
  content: "";
  /*絶対配置で位置を指定*/
  position: absolute;
  bottom: 0;
  left: 0;
  z-index: 2;
  /*色や形状*/
  background: rgba(90, 150, 255, 0.5);
  width: 100%;
  height: 0;
  /*アニメーション*/
  transition: 0.3s cubic-bezier(0.8, 0, 0.2, 1) 0s;
}

/*hoverした際の形状*/
.bgbottom:hover:before {
  height: 100%;
  background: rgba(90, 150, 255, 0.5);
}
/* ==== 背景が流れる（中央から横全体） ==== */
.bgcenterx:before {
  content: "";
  /*絶対配置で位置を指定*/
  position: absolute;
  top: 0;
  left: 0;
  z-index: 2;
  /*色や形状*/
  background: rgba(90, 150, 255, 0.5);
  width: 100%;
  height: 100%;
  /*アニメーション*/
  transition: transform 0.3s cubic-bezier(0.8, 0, 0.2, 1) 0s;
  transform: scale(0, 1);
  transform-origin: top; /* bottomでも同じです */
}

/*hoverした際の形状*/
.bgcenterx:hover:before {
  transform: scale(1, 1);
}
/* ==== 背景が流れる（中央から縦全体） ==== */
.bgcentery:before {
  content: "";
  /*絶対配置で位置を指定*/
  position: absolute;
  top: 0;
  left: 0;
  z-index: 2;
  /*色や形状*/
  background: rgba(90, 150, 255, 0.5);
  width: 100%;
  height: 100%;
  /*アニメーション*/
  transition: transform 0.3s cubic-bezier(0.8, 0, 0.2, 1) 0s;
  transform: scale(1, 0);
  transform-origin: center;
}

/*hoverした際の形状*/
.bgcentery:hover:before {
  transform: scale(1, 1);
}

/* ==== 背景が流れる（中央から外） ==== */
.bgcenterout:before {
  content: "";
  /*絶対配置で位置を指定*/
  position: absolute;
  top: 0;
  left: 0;
  z-index: 2;
  /*色や形状*/
  background: rgba(90, 150, 255, 0.5);
  width: 100%;
  height: 100%;
  /*アニメーション*/
  transition: transform 0.3s cubic-bezier(0.8, 0, 0.2, 1) 0s;
  transform: scale(0, 0);
  transform-origin: center;
}

/*hoverした際の形状*/
.bgcenterout:hover:before {
  transform: scale(1, 1);
}

/* ==== 背景が流れる（斜め） ==== */
.bgskew::before {
  content: "";
  /*絶対配置で位置を指定*/
  position: absolute;
  top: 0;
  left: -130%;
  /*色や形状*/
  background: rgba(90, 150, 255, 0.5);
  width: 120%;
  height: 100%;
  transform: skewX(-25deg);
}

/*hoverした時のアニメーション*/
.bgskew:hover::before {
  animation: skewanime 0.7s forwards; /*アニメーションの名前と速度を定義*/
}

@keyframes skewanime {
  0% {
    right: -10%; /*画面の見えていない左から右へ移動する終了地点*/
  }
  100% {
    left: 110%; /*画面の見えていない左から右へ移動する終了地点*/
  }
}

/*========= レイアウトのためのCSS ===============*/

body {
  vertical-align: middle;
  padding: 20px 0;
  text-align: center;
}

p {
  margin: 0 0 10px 0;
}
!

JS
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