Improve Compatibility with Browser Fallbacks issue

I dont understand How to solve this task? Can anyone help?

Your code so far

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

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:64.0) Gecko/20100101 Firefox/64.0.

Link to the challenge:

They just ask you to write before the background declaration another background declaration with a different value.

This is just in case if the browser doesn’t understand css variables .

Got it! thanks to you