css circles

HTML

<div class="container">

  <div class="circle circle1">
    <div class="circle-container">
      <a href="https://google.de" target="_blank">
        <span>Google</span>
      </a>
    </div>
  </div>
  
  <div class="circle circle2">
    <div class="circle-container">
      <a href="https://i-san.de" target="_blank">
        <span>I-SAN</span>
      </a>
    </div>
  </div>

</div>

CSS

.container {
  position: relative;
  width: 600px;
  height: 600px;
  margin: 15px auto;
  background-color: #dddddd;
  max-width: 100%;
}

.circle-container {
  width: 100%;
  height: 0;
  padding-bottom: 100%;
  position: relative;
}

.circle-container a {
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  background-color: cyan;
  border-radius: 50%;
  
  /* center content */
  display: flex;
  align-items: center;
  justify-content: center;
  align-content: center;
}

.circle-container a:hover {
  background-color: blue;
  color: white;
}

.circle {
  position: absolute;
  /* correct hitboxes */
  border-radius: 50%;
  overflow: hidden;
}

.circle1 {
  top: 20%;
  left: 20%;
  width: 20%;
}

.circle2 {
  top: 30%;
  left: 30%;
  width: 30%;
}

.circle2 a {
  background-image: url("http://lorempixel.com/400/400/cats/");
  background-position: center center;
  background-size: cover;
}