Firstly we need to query PHP variable and check if the browser supported output is ok:

I suggest this gets added after the include_once('lib/debug/debugger.php'); somewhere near the bottom of tiki-setup.php

if (strstr($_SERVER['HTTP_ACCEPT'], 'application/xhtml+xml')) {
 header("Content-type: application/xhtml+xml");
 } else {
 header("Content-type: text/html");
 }

Fingers crossed that should work.

Ive tested on a older 1.7 installation.

Damian