Background is not working,why\\\

Question

https://code.sololearn.com/WksT6EseZgOM/?ref=app

Answer 1

Try to remove or comment the CSS rule:

li { float:left; }

Answer 2

Use the clearfix hack:

ul::after {
content: "";
clear: both;
display: table;
}

(https://www.w3schools.com/css/css_float.asp)