@keyframes spinner {
  to {transform: rotate(360deg);}
}
body {
  padding: 30px 39px 39px 39px;
}

.spinner:before {
  content: '';
  box-sizing: border-box;
  position: absolute;
  top: 50%;
  left: 50%;
  width: 20px;
  height: 20px;
  margin-top: -10px;
  margin-left: -10px;
  border-radius: 50%;
  border-top: 2px solid #07d;
  border-right: 2px solid transparent;
  animation: spinner .6s linear infinite;
}

.output {
  font-family: Consolas, monaco, monospace;
  white-space: pre;
  margin-top: 4vh;
}

.error {
  color: red;
}