To force IE to edge mode (even on intranet sites, where IE would otherwise use compatibility mode), the server needs to send X-UA-Compatible as an HTTP header. Using a meta element in the document head doesn’t work reliably.
I found the answer buried in this Stack Overflow thread.
In my case, I was working on a WordPress theme, so I added
at the top of header.php.
UPDATE 3/13/2014: I have now verified the IIS method linked above.