Improve Compatibility with Browser Fallbacks how to do it

Tell us what’s happening:

Your code so far

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

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.102 Safari/537.36.

Link to the challenge:

You just have to add background: red; to the .red-box style

But I have tried it is not working

What do you mean? Show us the code please. The code that you showed does not contain background:red

please assist i added it but it was not still working