<div class="c">
  <input type="checkbox" id="faq-1">
  <h1><label for="faq-1">What Is This ?</label></h1>
  <div class="p">
    <p>This a very very simple accordion.</p>
  </div>
</div>
<div class="c">
  <input type="checkbox" id="faq-2">
  <h1><label for="faq-2">With Pure Css ?</label></h1>
  <div class="p">
    <p>Yes with pure CSS and HTML.</p>
  </div>
</div>
<div class="c">
  <input type="checkbox" id="faq-3">
  <h1><label for="faq-3">Where did you get inpiration ?</label></h1>
  <div class="p">
    <p>I was inpired by an article on css-tricks. <a href="https://css-tricks.com/the-checkbox-hack/">link to article</a>
    </p>
  </div>
</div>

@import url("https://fonts.googleapis.com/css?family=Poppins:400,400i,700");
*, *::after, *::before{
  margin: 0;
  padding: 0;
  box-sizing:border-box;
}
body{
  font-family: "Poppins", sans-serif;
}
div.c{
  position: relative;
  margin:2em;
}
input{
  position: absolute;
  left: 0;
  top: 0;
  height: 100%;
  width: 100%;
  opacity:0;
  visibility: 0;
}
h1{
  background:steelblue;
  color:white;
  padding:1em;
  position: relative;
}
label::before{
  content:"";
  display: inline-block;
  border: 15px solid transparent;
  border-left:20px solid white;
}
label{
  cursor: pointer;
  position: relative;
  display: flex;
  align-items: center;
}
div.p{
  max-height:0px;
  overflow: hidden;
  transition:max-height 0.5s;
  background-color: white;
  box-shadow:0 0 10px 0 rgba(0, 0, 0, 0.2);
}
div.p p {
  padding:2em;
}
input:checked ~ h1 label::before{
  border-left:15px solid transparent;
  border-top:20px solid white;
  margin-top:12px;
  margin-right:10px;
}
input:checked ~ h1 ~ div.p{
  max-height:100px;
}
a{
  color:steelblue;
}