Windows 10 introduces Edge as its default web browser, but Microsoft made a big deal out of keeping IE11 around for compatibility purposes. And by all accounts IE11 on Win 10 should be functionality identical to IE11 on Windows 7 or 8/8.1. We were surprised then to run into a problem on Win 10 that we did not see previously.

What we saw on a few websites, including an internal site and the Office 365 portal, was some graphics not displaying. The bottom (green) image here is correct, and as you can see the top one is missing the bell and other graphics. Also you can see how the Office 365 portal was missing the icons for various apps.

Again, we were only experiencing this using IE11 on Windows 10. All of our IE policies were applied to all our operating systems, so there should not have been any policy differences in IE11 due to the underlying OS. Turns out though a policy that seemingly has nothing to do with IE was the culprit.

Both of these websites, and no doubt others, actually use icon fonts to display the graphics in question. And on Windows 10 there’s a new policy – Computer Configuration > Policies > Admin Templates > System > Mitigation Options > Untrusted Font Blocking – which, if enabled, will block IE from downloading new fonts. Edge is unaffected.

Once we turned the policy off things went back to normal.