We were using the deprecated Microsoft Mobile Toolkit to detect mobile devices. Due to this IE11 was detecting the ContentType of the page as "text/xml" instead of "text/html". Instead of using something else to detect mobile devices, I decided to do the following in our aspx page's base page:
protected override void OnPreInit(EventArgs e)
{
base.OnPreInit(e);
HttpContext.Current.Response.Clear();
HttpContext.Current.Response.ContentType = "text/html";
}