How to solve this problem?

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; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/55.0.2883.87 UBrowser/ Safari/537.36.

Link to the challenge:

Not all browsers understand css code with variables. For such browsers right before the existing background we can add another background declaration with a most videly supported value, such as color name. All browsers will execute first line of code with color name. If they understand a code with css variables, they will execute code of a second line. If they don’t understand that code, they will leave a value of background from a first declaration.