htmlpurifier/library/HTMLPurifier/HTMLModule/Tidy/XHTML.php

<?php

class HTMLPurifier_HTMLModule_Tidy_XHTML extends
      HTMLPurifier_HTMLModule_Tidy
{
    
    public $name = 'Tidy_XHTML';
    public $defaultLevel = 'medium';
    
    public function makeFixes() {
        $r = array();
        $r['@lang'] = new HTMLPurifier_AttrTransform_Lang();
        return $r;
    }
    
}