devtools-docs/docs/elements-styles-files/tutorial.css

#computed_styles_test {
  border: 2px solid black;
  background-color: #EEE;
  padding: 4px;
  color: green;
  opacity: 0.8;
}

#test_styles {
  border: 2px solid black;
  background-color: #EEE;
  padding: 4px;
  color: green;
  -moz-opacity: 0.8;
  opacity: 0.8;
  worder: clue;
}

#test_styles::after {
  content: " (Inspect Me)";
}