twbs/bootstrap — pull request #14379 by electricessence (2014-08-12 20:19:54+00:00)
Title: Lack of background-color in gradients.

For some cases like IE9 (non-compat mode) where you can only have one filter applied at a time or any time a gradient filter is set as disabled (enabled=false) the background color is non-existent or gray.
There should be a valid approximation/fallback for these cases.