Question about using responsive design with bootstrap fluid containers

1

Tell us what’s happening:
I finished the question but I am confused to as 1. Why do we should nest all of our HTML in a div element with the class container-fluid. 2. Why the class “.container-fluid” does not appear in our style tag. 3. If I add the code, that the challenge says can add bootstrap to any app, will that code work for almost any normal web app I’m creating without editing it & does the div tag and the class we added to the div tag have anything to do with that bootstrap code?

Your code so far

<link href="https://fonts.googleapis.com/css?family=Lobster" rel="stylesheet" type="text/css">
<style>
  .red-text {
    color: red;
  }

  h2 {
    font-family: Lobster, Monospace;
  }

  p {
    font-size: 16px;
    font-family: Monospace;
  }

  .thick-green-border {
    border-color: green;
    border-width: 10px;
    border-style: solid;
    border-radius: 50%;
  }

  .smaller-image {
    width: 100px;
  }
</style>
<div class="container-fluid">
<h2 class="red-text">CatPhotoApp</h2>

<p>Click here for <a href="#">cat photos</a>.</p>

<a href="#"><img class="smaller-image thick-green-border" src="https://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="/submit-cat-photo">
  <label><input type="radio" name="indoor-outdoor"> Indoor</label>
  <label><input type="radio" name="indoor-outdoor"> Outdoor</label>
  <label><input type="checkbox" name="personality"> Loving</label>
  <label><input type="checkbox" name="personality"> Lazy</label>
  <label><input type="checkbox" name="personality"> Crazy</label>
  <input type="text" placeholder="cat photo URL" required>
  <button type="submit">Submit</button>
</form>
</div>```
**Your browser information:**

Your Browser User Agent is: ```Mozilla/5.0 (Windows NT 10.0; rv:56.0) Gecko/20100101 Firefox/56.0```.

**Link to the challenge:**
https://www.freecodecamp.org/challenges/use-responsive-design-with-bootstrap-fluid-containers