What does it mean when you add \9 to the end of a CSS color?

Question

Looking at Bootstrap 2.3 I see this in a few places:

.btn:active,
.btn.active {
  background-color: #cccccc \9;
}

Does anyone know what it means to add the \9 to the end of the color?

Answer 1

It's a hack for old version of Internet Explorer. This basically means that this rule will apply on IE7-9, and ignored by all other browsers.

Answer 2

\9 is a "CSS hack" specific to Internet Explorer 7, 8, & 9.

This simply means that the one specific line of CSS ending with a \9; in place of the ; is only valid in IE 7, 8, & 9.

In your example, background-color: #cccccc \9; will only apply to IE.