https://www.freecodecamp.org/learn/responsive-web-design/basic-css/use-a-css-class-to-style-an-element
Use a CSS Class to Style an Element
Classes are reusable styles that can be added to HTML elements.
Here's an example CSS class declaration:
<style> .blue-text { color: blue; } </style>
You can see that we've created a CSS class called blue-text within the <style> tag. You can apply a class to an HTML element like this: <h2 class="blue-text">CatPhotoApp</h2>. Note that in your CSS style element, class names start with a period. In your HTML elements' class attribute, the class name does not include the period.
TEST
Inside your style element, change the h2 selector to .red-text and update the color's value from blue to red.
Give your h2 element the class attribute with a value of red-text.
<style>
.red-text {
color: red;
}
</style>
<h2 class="red-text">CatPhotoApp</h2>
<main>
<p>Click here to view more <a href="#">cat photos</a>.</p>
<a href="#"><img src="https://bit.ly/fcc-relaxing-cat" alt="A cute orange cat lying on its back."></a>
<div>
<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>
</div>
<form action="https://freecatphotoapp.com/submit-cat-photo">
<label><input type="radio" name="indoor-outdoor" checked> Indoor</label>
<label><input type="radio" name="indoor-outdoor"> Outdoor</label><br>
<label><input type="checkbox" name="personality" checked> Loving</label>
<label><input type="checkbox" name="personality"> Lazy</label>
<label><input type="checkbox" name="personality"> Energetic</label><br>
<input type="text" placeholder="cat photo URL" required>
<button type="submit">Submit</button>
</form>
</main>