X-UA-Compatible META tag ignored because document mode is already finalized
|Knowledge Article Number||000175466|
If you're getting the warning, “HTML1115: X-UA-Compatible META tag ('IE=8') ignored because document mode is already finalized” it's because Internet Explorer has a strict implementation of the x-ua-compatible META tag in this scenario. Specifically, it's required to be directly at the top of the Head element.
Since you can't control this at the Visualforce layer, it's recommended that you set the header as follows within the controller constructor: