I dont know the theme you using but an round about way is change it from xhtml to html5 atleast for the header.
find
Code:
if (isset($INSTALLER09['xhtml_strict'])) { //== Use strict mime type/doctype
//== Only if browser/user agent supports xhtml
if (isset($_SERVER['HTTP_ACCEPT']) && stristr($_SERVER['HTTP_ACCEPT'],'application/xhtml+xml') && ($INSTALLER09['xhtml_strict'] === 1 || $INSTALLER09['xhtml_strict'] == $CURUSER['username'])) {
header('Content-type:application/xhtml+xml; charset='.charset());
$doctype ='<?xml version="1.0" encoding="'.charset().'"?>'.'<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">'.'<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">';
}
}
if (!isset($doctype)) {
header('Content-type:text/html; charset='.charset());
$doctype = '<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"'.'"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">'.'<html xmlns="http://www.w3.org/1999/xhtml">';
}
replace with
Code:
/*
if (isset($INSTALLER09['xhtml_strict'])) { //== Use strict mime type/doctype
//== Only if browser/user agent supports xhtml
if (isset($_SERVER['HTTP_ACCEPT']) && stristr($_SERVER['HTTP_ACCEPT'],'application/xhtml+xml') && ($INSTALLER09['xhtml_strict'] === 1 || $INSTALLER09['xhtml_strict'] == $CURUSER['username'])) {
header('Content-type:application/xhtml+xml; charset='.charset());
$doctype ='<?xml version="1.0" encoding="'.charset().'"?>'.'<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">'.'<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">';
}
}
*/
if (!isset($doctype)) {
header('Content-type:text/html; charset='.charset());
$doctype = '<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"'.'"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">'.'<html xmlns="http://www.w3.org/1999/xhtml">';
}
saves you countless hours debugging the theme to figure why xhtml errors out.