What does an underscore “_” mean in CSS?

Question

I just found the following snippet in a CSS file:

position: fixed;
_position: absolute;

What does this underline mean in front of the second

Answer 1

It's one of a number of CSS "hacks" used to target specific versions of Internet Explorer.

selector {
    property: value;   /* all browsers */
    property: value\9; /* < IE9 */
    *property: value;  /* < IE8 */
    _property: value;  /* < IE7 */
}

Generally speaking you should avoid CSS hacks in favor of conditional classes on HTML.

Answer 2

This is an old CSS-Hack for IE5, 5.5 & 6.
All browser will display the position:fixed while IE5 - 6 use the _position, so it display it absolute.

But note: This CSS won't validate! And it won't work for IE5/MAC

Answer 3

This is a way to give alternative directives to WinIE browsers, since they don't support certain features of the latest CSS definitions. Other browsers will ignore the whole definition (e.g. _position: relative), while WinIE will treat it as position: relative.