Why do we need multiple background color rules?

I am new to Css and I am having a hard time finding this particular case on the internet, so I am posting a question here.

I am working on an existing codebase and I see something like this.

li {
  background-color: #000 \9;
  background-color: rgba(0, 0, 0, 0);
}

      

        
        
        
      

    

I don’t understand the point \9

. But it seems to me that the rules are duplicates and I have to remove one of them.

Could you explain \9

and remove one rule? Thank.

The reason I am asking is because I am converting some css files to fewer files and \9

causing a compilation problem.