Learn CSS Variables by Building a City Skyline - Step 10

Hi guys, i need help to understand what am doing wrong?
Your code so far

/* file: index.html */
<!DOCTYPE html>
<html lang="en">    
<head>
  <meta charset="UTF-8">
  <title>City Skyline</title>
  <link href="styles.css" rel="stylesheet" />
</head>

<body>
  <div class="background-buildings">
   <div class="bb1"></div> 
  </div>
.bb1 {
width: 10%;
}
</body>
</html>
/* file: styles.css */
* {
border: 1px solid black;
box-sizing: border-box;
}

body {
height: 100vh;
margin: 0;
overflow: hidden;
}

.background-buildings {
width: 100%;
height: 100%;
}
  
  **Your browser information:**

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.0.0 Safari/537.36

Challenge: Learn CSS Variables by Building a City Skyline - Step 10

Link to the challenge:

Hmmm…

Why are you declaring a class in your html code?