The HTTP content-type "application/xhtml+xml" causes IE to try to download the page instead of displaying it. Firefox seems to have no problem with that.
Tested on Windows 7 x64 with IE 8.0
Additional Information
I modified the http_content_headers() function in http_api.php to:
/**
Set content-type headers.
*/
function http_content_headers() {
if ( !headers_sent() ) {
if (stristr($_SERVER[HTTP_ACCEPT], "application/xhtml+xml")) {
header( 'Content-Type: application/xhtml+xml; charset=UTF-8' );
} else {
header( 'Content-Type: text/html; charset=UTF-8' );
}
// Disallow Internet Explorer from attempting to second guess the Content-Type
// header as per http://blogs.msdn.com/ie/archive/2008/07/02/ie8-security-part-v-comprehensive-pro
header( 'X-Content-Type-Options: nosniff' );
}
}
Fix 0012641: Internet Explorer does not support xhtml+xml MIME type
Internet Explorer prior to version 9 does not support the
application/xhtml+xml MIME type and will therefore not render pages
served up in this way.
We should check the "Accept" header sent by the browser to ensure that
the browser can handle application/xhtml+xml before we use this MIME
type. Otherwise the default should be to use the text/html MIME type for
older browsers.
Commit f081ed1f0171435e5a61880d46ca2e95a8c6b01e accidentally referenced
$_SERVER[HTTP_ACCEPT] where HTTP_ACCEPT is not a valid constant. It
should be $_SERVER['HTTP_ACCEPT'] instead (ie. the array key is a
string).
