IE8 and older don’t play well with the ‘rgba’ property (it’s the alpha setting). This can be a problem for the slideshow captions that are set to black background with 50% alpha by default starting on line 224 in slideshow.css. A workable fallback is to duplicate the background line in the custom.css file with a rgb only version preceding the rgba setting.
background: rbg (0, 0, 0);
background: rbga (0, 0, 0, 0.5);
It might make sense to work this in to a future update.