CSS Cross Browser Transparent Background

The startColor and endColor parameters accept:
An Integer that specifies or receives the color value that can range from 0 (transparent) to 4294967295 (opaque white).
http://msdn.microsoft.com/en-us/library/ms532929(v=vs.85).aspx

You can also use a startColorStr and/or endColorStr which accept:
A String that specifies or receives a value that can range from #FF000000 to #FFFFFFFF.

So you can specify the colors in „#RRGGBB“ (as in the example) or „#AARRGGBB“ format.

Color is expressed in #AARRGGBB format like this:
AA is the alpha hexadecimal value
RR is the red hexadecimal value
GG is the green hexadecimal value
BB is the blue hexadecimal value.
The alpha value controls the opacity (00 is transparent, FF is opaque).

The default value is #FF0000FF (opaque blue) and if you pass a value that is out of range then it defaults to it.
http://msdn.microsoft.com/en-us/library/ms532930(v=vs.85).aspx

Don’t forget that the object must have a layout for the filter to render.
http://msdn.microsoft.com/en-us/library/ie/ms530752(v=vs.85).aspx