Hidden resources are not dimmed correctly in IE in certain standards modes

From what I remember, IE applies different standards modes (e.g. Quirks Mode) depending on whether it detects the target as inside your LAN. This can be confirmed by opening the Developer Tools' Console when refreshing the page. I get

HTML1202: http://kumquat.lancs.ac.uk/moodle/course/view.php?id=2 is running in Compatibility View because 'Display intranet sites in Compatibility View' is checked. 

view.php?id=2

I'm seeing this with the following:
My hostname: kumquat.lancs.ac.uk
URL of my Moodle: http://kumquat.lancs.ac.uk/moodle

This is causing me to get the following:
Browser Mode: IE9 Compat View
Document Mode: IE7 Standards

As a result, when I'm in editing mode and hide a resource, it doesn't actually change the opacity (or rather, the change in opacity doesn't do anything).

This breakage can be confirmed by using the IE developer tools to change the Document Mode to either 'IE7 standards' or 'IE8 standards'

Not sure whether the best approach to fixing this is to change the base theme to force the standards mode for infranet sites, or find a solution for IE7