Custom post type not shown properly on IE 9 – any ideas?

So with any real browser, even with IE 10 and 11, my room pages are shown nicely (like this one: http://freiburg-innenstadthotel.de/room/einzelzimmer-comfort/) – at least as far as I know of. Only in IE 9 it looks like that – any ideas how I can fix that? At least to a degree that it is "readable" for IE 9-users?