Why do we need multiple background-color rules? [duplicate]

Question

I'm a novice in Css and it's difficult to search for this specific case on the internet, so I post a question here.

I'm working on an existing code base and I see something like this.

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

I don't understand the meaning of \9. But it looks to me the rules are duplicates and I should remove one of them.

Could you please explain the \9 and should I remove one rule? Thanks.

The reason I'm asking because I'm converting some css files to less files and \9 is causing a compilation problem.

Answer 1

\9 is an old Internet Explorer hack, which makes it work in IE only (8 and lower) rgba is the way to go you can safely delete the \9 rule

Answer 2

This is a CSS hack for IE 7, 8, & 9.

To find out more, please check this link.