html, body {
  margin: 0;
  width: 100%;
  height: 100%;
}

#can1 {
  width: 100vw;
  height: 100vh;
  display: block;
}

#div1 {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
}
#div4 {
  position: absolute;
  bottom: 10px;
  right: 10px;
  width: 200px;
 }

#b0, #b1, #b2, #b3 {
  border: 0;
  width: 180px;
  line-height: 2.5;
  padding: 0 20px;
  font-size: 1rem;
  text-align: center;
  color: #fff;
  text-shadow: 2px 2px 2px #000;
  border-radius: 10px; 
  background-image: linear-gradient(to top left, rgba(0, 0, 0, 3), rgba(0, 0, 0, .2), rgba(0, 0, 0, 0));
  box-shadow: inset 2px 2px 3px rgba(255, 255, 255, .6), inset -2px -2px 3px rgba(0, 0, 0, .6);
}

#b1:active, #b2:active, #b3:active, #b0:active {
  box-shadow: inset -2px -2px 3px rgba(255, 255, 255, .6), inset 2px 2px 3px rgba(0, 0, 0, .6);
}