How to solve this problem?

Tell us what’s happening:

Your code so far


<style>
  .background{
    color:red;
  }
  :root {
    --red-color: red;
  }
  .red-box {
    
    background: var(--red-color);
    height: 200px;
    width:200px;
  }
</style>
<div class="red-box"></div>

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.87 UBrowser/7.0.185.1002 Safari/537.36.

Link to the challenge:
https://learn.freecodecamp.org/responsive-web-design/basic-css/improve-compatibility-with-browser-fallbacks

The fallback will be a property inside the red-box class. Instead, you have created a background class with a color property of red.

Not all browsers understand css code with variables. For such browsers right before the existing background we can add another background declaration with a most videly supported value, such as color name. All browsers will execute first line of code with color name. If they understand a code with css variables, they will execute code of a second line. If they don’t understand that code, they will leave a value of background from a first declaration.