twitter/bootstrap — pull request #2991 by M-O-Z-G (2012-04-08 18:16:18+00:00)
Title: Make "#translucent > .background" truly translucent in IE.

Now translucent mixin use HSLA color code for add alpha to color. It doesn't work in IE7-8 doesn't understand the alpha transparency in HSLA mode. For this purpose, the code added ARGB conversion model, and creating a filter for IE.
Unfortunately, because a background in filter not clipping with border-radius in IE9, appended to the hagfish style leads radius to 0 in this version.