Code should look okay by default.

I think most people are going to use highlight.js or Prism or something if they want to style their code blocks but it wouldn't hurt to make them look okay out of the box, even with no syntax highlighting.

What I've written here is probably long enough, but adding this final sentence can't hurt.

function tick() {
  const element = (
    <div>
      <h1>Hello, world!</h1>
      <h2>It is {new Date().toLocaleTimeString()}.</h2>
    </div>
  );
  ReactDOM.render(element, document.getElementById('root'));
} 

Hopefully that looks good enough to you.

We still need to think about stacked headings though.

Let's make sure we don't screw that up with <code>h4</code> elements, either.

Phew, with any luck we have styled the headings above this text and they look pretty good.

Let's add a closing paragraph here so things end with a decently sized block of text. I can't explain why I want things to end that way but I have to assume it's because I think things will look weird or unbalanced if there is a heading too close to the end of the document.

What I've written here is probably long enough, but adding this final sentence can't hurt.