Button Background image 100%

HTML

<button id="test"></button>

<h1><a href="http://stackoverflow.com/questions/35014898/image-background-buttom">Question Stackoverflow</a></h1>

CSS

#test{
  position: relative;
  z-index:99; 
  margin-top: 20px;
  width: 100px;
  height: 100px;
  background-image: url(https://i.stack.imgur.com/4PPsT.png);
  background-repeat: no-repeat;
}