Responsive CSS round button

HTML

<div class="round-button">
  <div class="round-button-border"></div>
  <div class="round-button-circle"><a href="http://example.com" class="round-button">Button!!</a></div>
</div>

CSS

.round-button {
  width: 25%;
}

.round-button-circle {
  width: 100%;
  height: 0;
  padding-bottom: 100%;
  border-radius: 50%;
  border: 10px solid #98a1a4;
  overflow: hidden;
}

.round-button-circle {
  width: 100%;
  height: 0;
  padding-bottom: 100%;
  border-radius: 50%;
  border: 10px solid #98a1a4;
  overflow: hidden;
  background: transparent;
  box-shadow: 0 0 3px gray;
}

.round-button-circle:hover {
  background: #30588e;
}

.round-button a {
  display: block;
  float: left;
  width: 100%;
  padding-top: 50%;
  padding-bottom: 50%;
  line-height: 1em;
  margin-top: -0.5em;
  text-align: center;
  color: #e2eaf3;
  font-family: Verdana;
  font-size: 1.2em;
  font-weight: bold;
  text-decoration: none;
}