SOURCE

<h1>HTML debugging examples</h1>

    <p>What causes errors in HTML?

    <ul>
      <li>Unclosed elements: If an element is <strong>not closed properly, then its effect can spread to areas you didn't intend

      <li>Badly nested elements: Nesting elements properly is also very important for code behaving correctly. <strong>strong <em>strong emphasised?</strong> what is this?</em>

      <li>Unclosed attributes: Another common source of HTML problems. Let's look at an example: <a href="https://www.mozilla.org/>link to Mozilla homepage</a> 
    </ul>