How to improve compatability with browser fallbacks

Can’t figure out how to improve the compatbility with browser fallbacks. In the Basic CSS test. I tried to get a hint but there is no valid page and there is no watch a video option. Any help would be greatly appreciated.

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

What did you try? This is just the code from the lesson.

What do you not understand about the lesson?
In the red-box class declaration it’s showing you background property has a value that is a variable. But not all browsers support variables so it wouldn’t know what to do with that line.
How would you define a declaration with a background property that has a value of red?