Use HTML5 to Require a Field

Summary

You can require fields to be filled out before a form is submitted.
If you wanted to make a text input field required, you can just add the attribute required within your input element.

Final Code

<h2>CatPhotoApp</h2>
<main>
  <p>Click here to view more <a href="#">cat photos</a>.</p>

  <a href="#"><img src="https://www.bit.ly/fcc-relaxing-cat" alt="A cute orange cat lying on its back."></a>

  <p>Things cats love:</p>
  <ul>
    <li>cat nip</li>
    <li>laser pointers</li>
    <li>lasagna</li>
  </ul>

  <p>Top 3 things cats hate:</p>
  <ol>
    <li>flea treatment</li>
    <li>thunder</li>
    <li>other cats</li>
  </ol>
  
  <form action="https://www.freecatphotoapp.com/submit-cat-photo">
    <input type="text" placeholder="cat photo URL" required>
    <button type="submit">Submit</button>
  </form>
</main>