monaco-editor/website/playground/new-samples/creating-the-editor/syntax-highlighting-for-html-elements/sample.html

<pre id="code" data-lang="text/css" style="width:500px;">
/* Some example CSS */

@import url("something.css");

body {
  margin: 0;
  padding: 3em 6em;
  font-family: tahoma, arial, sans-serif;
  color: #000;
}

#navigation a {
  font-weight: bold;
  text-decoration: none !important;
}

h1 {
  font-size: 2.5em;
}

h2 {
  font-size: 1.7em;
}

h1:before, h2:before {
  content: "some contents";
}

code {
  font-family: courier, monospace;
  font-size: 80%;
  color: #418A8A;
}
</pre>