The problem is that IE is displaying all intranet sites in compatibility mode by default. https://developers.google.com/web-toolkit/doc/latest/DevGuideIE9
This means that even if the client is using IE9, he might see the content rendered with IE7 standards. This is problematic, because until recently we were still supporting IE7, but we are moving away from that, rewriting and simplifying our code, and thus introducing more incompatibilities with older browsers.
You can read
XWIKI-8284 for certain solutions regarding changing preferences in IE, but there is also another solution provided by James P in XWIKI-8891 where we could modify the header (making the change in htmlheader.vm) and adding:
For reference, there are several mails on users mailinglist, like:
from user that encounters problems when Compatibility mode is enabled.