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

header(“X-UA-Compatible: IE=Edge”);

at the top of header.php.

You can also do it at the web server level with Apache or IIS (I haven’t tested either of those methods).

UPDATE 3/13/2014: I have now verified the IIS method linked above.