Your master pages are probably default IIS and are either http-equiv="X-UA-Compatible" content="IE=8" or "IE=10"
If you must continue as-is then use an .aspx HTML file which loads with a meta tag that instructs IE to render in EDGE mode, and your XHTML page in an iframe. Your master page is probably already IE can only set the rendering engine once per page load. Once set it cannot be changed and thus all pages loaded through an iframe are forced to EDGE mode.
I would try
<!doctype html>, look to see what is no longer rendering, take a search at that over at http://caniuse.com/ or scan with Microsoft's tool to check for old depreciated tags over at https://developer.microsoft.com/en-us/microsoft-edge/tools/staticscan/
You may also enjoy - or start crying - reading What happens in Quirks Mode? - the choice is yours!