Improve Compatibility with Browser Fallbacks and help to get output for this and explanation

Tell us what’s happening:
which class to add background:red; please explain and help to get output for this program completely please

Your code so far

  :root {
    --red-color: red;

  .red-box {
    background: var(--red-color);
    height: 200px;
<div class="red1-box"></div>
<div class="red-box"></div>

Your browser information:

User Agent is: Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36.

.red-box before the current background property. Click “Run the Test”.