Use opacity instead of filter if the browser is IE9 or higher
IE9 and IE10 are supporting the opacity css property. It would be better to use it instead of filters. You might use -ms-filter if the browser is IE8, because the original filter property might cause issues.
cool. I'll add a workaround as done the same as Morpheus
+1 This is also an issue if the UserAgent string was changed - I had to do this with Chrome to get through a nasty proxy. Not an issue with the technique used by Morpheus.
Using the MS filter caused problems in my code with IE9; the problem was fixed by editing Bonzo to use opacity instead.
Switching between opaque and transparent caused my div to become permanently transparent. I can provide a minimal example if needed.