Improve Compatibility with Browser Fallbacks...>>

Tell us what’s happening:
I couldn’t understand, what I am asked to do. I want help with solve.

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; rv:66.0) Gecko/20100101 Firefox/66.0.

Here the background is coloured using a variable, but some browsers don’t use variables so to make sure that those browsers will use the right color for the background you need to add a new background rule that doesn’t use variables