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HTML

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About the <head>

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Insecure Resource

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CSS

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CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <ul class="accordion__list">
  <li class="accordion__item">
    <div class="accordion__head">
      <p>アコーディオン1の開閉ボタン</p>
    </div>
    <div class="accordion__body">
      <p>アコーディオン1の中身</p>
    </div>
  </li>
  <li class="accordion__item">
    <div class="accordion__head">
      <p>アコーディオン2の開閉ボタン</p>
    </div>
    <div class="accordion__body">
      <p>アコーディオン2の中身</p>
    </div>
  </li>
  <li class="accordion__item">
    <div class="accordion__head">
      <p>アコーディオン3の開閉ボタン</p>
    </div>
    <div class="accordion__body">
      <p>アコーディオン3の中身</p>
    </div>
  </li>
</ul>
!

CSS 

              
                /*
  アコーディオンそのものの横幅と余白を指定。
  開閉動作に影響なし。
*/
.accordion__list {
  width: 500px;
  padding: 10px;
}

/*
  アコーディオンの2つ目以降、上に余白を取る。
  開閉動作に影響なし。
*/
.accordion__item:nth-child(n+2) {
  margin-top: 10px;
}

/*
  アコーディオンの開閉ボタンの装飾。
  position: relative;が無いと、
  「+」「-」の位置がずれるので注意。
*/
.accordion__head {
  position: relative;
  width: 100%;
  padding: 10px;
  color: #fff;
  background: #019ac6;
  cursor: pointer;
}

/*
  アコーディオンの開閉ボタンの右側に「+」を表示。
*/
.accordion__head::after {
  content: "+";
  position: absolute;
  top: 8px;
  right: 10px;
}

/*
  アコーディオンが開いた際に、
  開閉ボタンの右側に「+」を「-」に変更する。
*/
.accordion__item.active .accordion__head::after {
  content: "-";
  right: 13px;
}

/*
  アコーディオンの閉じている部分の装飾。
  height: 0; overflow: hidden;にすることで、
  通常時は表示されないようにする。
*/
.accordion__body {
  overflow: hidden;
  width: 100%;
  height: 0;
  padding: 0 10px;
  background: #e8e8e8;
  transition: all .25s ease;
}

/*
  アコーディオンが開いた際に、
  heightを0からautoに、paddingの上下を0から10に。
*/
.accordion__item.active .accordion__body {
  height: auto;
  padding: 10px;
}
!

JS 

              
                // 厳格モードで実行
"use strict";

// アコーディオンパーツ全体と、開閉ボタンとなる部分のDOMを変数に格納。
const accordionItem = document.getElementsByClassName("accordion__item");
const accordionBtn = document.getElementsByClassName("accordion__head");

// 開閉ボタンがクリックされたときの処理。
// 開閉ボタンの数だけ処理できるようfor文を回す。
// accordionBtn[i]（i部分には何番目のアコーディオンボタンか、の数字が入る）がクリックされた際、
// accordionItem[i]（アコーディオンボタンと同じ数字のアコーディオンパーツ）に
// activeというcssのclassを付与する。
for(let i = 0; i < accordionBtn.length; i++) {
    accordionBtn[i].addEventListener("click", function() {
        accordionItem[i].classList.toggle("active");
    });
}
!
999px

Console