https://www.freecodecamp.org/learn/responsive-web-design/basic-css/use-css-selectors-to-style-elements
Use CSS Selectors to Style Elements
With CSS, there are hundreds of CSS properties that you can use to change the way an element looks on your page.
When you entered <h2 style="color: red;">CatPhotoApp</h2>, you were styling that individual h2 element with inline CSS, which stands for Cascading Style Sheets.
That's one way to specify the style of an element, but there's a better way to apply CSS.
At the top of your code, create a style block like this:
<style> </style>
Inside that style block, you can create a CSS selector for all h2 elements. For example, if you wanted all h2 elements to be red, you would add a style rule that looks like this:
<style> h2 { color: red; } </style>
Note that it's important to have both opening and closing curly braces ({ and }) around each element's style rule(s). You also need to make sure that your element's style definition is between the opening and closing style tags. Finally, be sure to add a semicolon to the end of each of your element's style rules.
TEST
Delete your h2 element's style attribute, and instead create a CSS style block. Add the necessary CSS to turn all h2 elements blue.
<style>
h2 {
color: blue;
}
</style>
<h2>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>