Error: CSS: background: / is an incorrect operator

Solution 1:

If we refer to the formal syntax:

The / is the separation between background-position and background-size (this one being optional) so the correct syntax is:

 background: url(...) 100% 0/4% no-repeat;

Where background-position:100% 0 and background-size:4%

Note that background-size should always be specified with background-position when using shorthand syntax. You cannot specify the size without position but you can specify position without size:

 background: url(...) 100% 0 no-repeat;

Relatad question: Issues with "background-position" in "background" shorthand property