It looks like a variable is being used to set the background color of the .red-box class. Let’s improve our browser compatibility by adding another background declaration right before the existing declaration and set its value to red.
Which means if the website fails to set the red box to the color red, you should add a background declaration (a fallback) to improve the compatibility of the browser.
Now you have to add a second background: ; property that is set to red - to your .red-box class - instead of the variable.