code_note

<div class="readmore">
  <input id="readmore" type="checkbox">
  <label for="readmore"></label>
  <div class="readmore-content">
    <p>ここにコンテンツ内容を入力します。</p>
  </div>
</div>

.readmore {
  position: relative;
  margin: 50px auto 0;
  padding: 0 0 75px;
}

.readmore label {
  position: absolute;
  display: table;
  left: 50%;
  bottom: 0;
  margin: 0 auto;
  width: 200px;
  padding: 10px 0;
  color: #fff;
  text-align: center;
  border-radius: 5px;
  background-color: #333;
  transform: translateX(-50%);
  cursor: pointer;
  z-index: 1;
}

.readmore label::before{
  content: '続きを見る';
}

.readmore input[type="checkbox"]:checked ~ label::before {
  content: '元に戻す';
}

.readmore input[type="checkbox"]{
  display: none;
}

.readmore-content {
  position: relative;
  height: 200px;
  overflow: hidden;
}

.readmore input[type="checkbox"]:checked ~ .readmore-content {
  height: auto;
}

inputのidとlabelのforで、これらは同じ値を付ける必要があります。

コンテンツ枠をグラデーションで隠す

.readmore-content::before {
  position: absolute;
  display: block;
  content: "";
  bottom: 0;
  left: 0;
  width: 100%;
  height: 75px;
  background: linear-gradient( rgba(255,255,255,0) 0%, rgba(255,255,255,0.8) 50%, #fff 100%);
}

.readmore input[type="checkbox"]:checked ~ .readmore-content::before {
  display: none;
}

cssだけでアコーディオンを作る

コンテンツ枠をグラデーションで隠す