We want IE11 to emulate IE10 so we're going to add a X-UA-Compatible directive to IE=EmulateIE10. But if our server is hit by IE9, how will IE9 interpret that directive?
Looking at https://msdn.microsoft.com/en-us/library/jj676915%28v=vs.85%29.aspx, IE9 will operate in IE9 Standards mode if it sees the X-UA-Compatable set to IE10)
("If the browser supports the header, but not any of the specified document modes, it will use the highest supported document mode to display the webpage.")
You can check this (if you have IE 9 lying around, which thankfully I do not) by going to the page and bringing up developer tools to see what mode it was set to and why (well, assuming developer tools in IE 9 worked the same way as it does with 10+)