<!doctype html>
<meta charset="utf-8">
<style>
  #button {
    width: 400px;
    height: 400px;
    margin: 100px;
    padding: 0;
    border: 0;
    background-color: purple;
    vertical-align: top;
  }
  #button > div {
    width: 100px;
    height: 100px;
    background-color: green;
  }
</style>
<button id="button">
  <div></div>
</button>
