<body>
  <nav class="accordion arrows">
    <header class="box">
      <label for="acc-close" class="box-title">Accordion menu</label>
    </header>
    <input type="radio" name="accordion" id="cb1" />
    <section class="box">
      <label class="box-title" for="cb1">Readme</label>
      <label class="box-close" for="acc-close"></label>
      <div class="box-content">Click on an item to open. Click on its header or the list header to close.</div>
    </section>
    <input type="radio" name="accordion" id="cb2" />
    <section class="box">
      <label class="box-title" for="cb2">Read me too</label>
      <label class="box-close" for="acc-close"></label>
      <div class="box-content">Add the class 'arrows' to nav.accordion to add dropdown arrows.</div>
    </section>
    <input type="radio" name="accordion" id="cb3" />
    <section class="box">
      <label class="box-title" for="cb3">Item 3</label>
      <label class="box-close" for="acc-close"></label>
      <div class="box-content">Vestibulum ante ipsum primis in faucibus orci luctus et ultrices posuere cubilia Curae; Quisque finibus tristique nisi, maximus ullamcorper ante finibus eget.</div>
    </section>

    <input type="radio" name="accordion" id="acc-close" />
  </nav>
</body>
body {
  height: calc(100% - 20px);
  width: calc(100% - 20px);
  margin: 0;
  padding: 10px;
  display: flex;
  background: #f2f2f2;
  color: rgba(0,0,0,.87);
  font-family: 'Roboto', sans-serif;
}
.accordion {
  margin: auto;
  width: 400px;
}
.accordion input {
  display: none;
}
.box {
  position: relative;
  background: white;
    height: 64px;
    transition: all .15s ease-in-out;
}
.box::before {
    content: '';
    position: absolute;
    display: block;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    pointer-events: none;
    box-shadow: 0 -1px 0 #e5e5e5,0 0 2px rgba(0,0,0,.12),0 2px 4px rgba(0,0,0,.24);
}
header.box {
  background: #00BCD4;
  z-index: 100;
  cursor: initial;
  box-shadow: 0 -1px 0 #e5e5e5,0 0 2px -2px rgba(0,0,0,.12),0 2px 4px -4px rgba(0,0,0,.24);
}
header .box-title {
  margin: 0;
  font-weight: normal;
  font-size: 16pt;
  color: white;
  cursor: initial;
}
.box-title {
  width: calc(100% - 40px);
  height: 64px;
  line-height: 64px;
  padding: 0 20px;
  display: inline-block;
  cursor: pointer;
  -webkit-touch-callout: none;-webkit-user-select: none;-khtml-user-select: none;-moz-user-select: none;-ms-user-select: none;user-select: none;
}
.box-content {
  width: calc(100% - 40px);
  padding: 30px 20px;
  font-size: 11pt;
  color: rgba(0,0,0,.54);
  display: none;
}
.box-close {
  position: absolute;
  height: 64px;
  width: 100%;
  top: 0;
  left: 0;
  cursor: pointer;
  display: none;
}
input:checked + .box {
  height: auto;
  margin: 16px 0;
    box-shadow: 0 0 6px rgba(0,0,0,.16),0 6px 12px rgba(0,0,0,.32);
}
input:checked + .box .box-title {
  border-bottom: 1px solid rgba(0,0,0,.18);
}
input:checked + .box .box-content,
input:checked + .box .box-close {
  display: inline-block;
}
.arrows section .box-title {
  padding-left: 44px;
  width: calc(100% - 64px);
}
.arrows section .box-title:before {
  position: absolute;
  display: block;
  content: '\203a';
  font-size: 18pt;
  left: 20px;
  top: -2px;
  transition: transform .15s ease-in-out;
  color: rgba(0,0,0,.54);
}
input:checked + section.box .box-title:before {
  transform: rotate(90deg);
}

/* Accordion menu with only html and CSS for reddit.com/r/css/comments/491w3v/challenge_2016_1_checkbox_hackery/ */

External CSS

  1. //fonts.googleapis.com/css?family=Roboto:300,400

External JavaScript

This Pen doesn't use any external JavaScript resources.