c9-core/plugins/c9.fs/mock/test.css

HTML,
H1 {
  background: orange;
  color: white;
  padding: 15px;
  font-size : 30px;
  border-radius: 6px;
  margin: 21px;
}

HTML{
  background: brown url("example.png");
  margin : 10px;
}

#hi{
    border: 2px solid red;
}