12.6 through 12.9 use a Meta Tag at the top of the login.htmp, and styles.htmpl files to enforce IE7 compatibility mode.
This restricts the ability to use newer formatting and document standards in Knowledge Documents that would display better in Chrome, Safari (and iOS Safari) and Firefox.
If compatibility mode is turned off in IE or those tags are removed, then there may be issues with using Service Desk.
Also the certification matrix for 12.9 mentions it is certified with IE 10 in Compatibility mode only. IE 11 is now available.
This idea is to consider certifying IE 10 fully and IE 11 without compatibility mode.