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

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.