en.javascript.info/2-ui/1-document/14-styles-and-classes/2-create-notification/source.view/index.css

.notification {
  position: fixed;
  z-index: 1000;
  padding: 5px;
  border: 1px solid black;
  font: normal 20px Georgia;
  background: white;
  text-align: center;
}

.welcome {
  background: red;
  color: yellow;
}