Attr.AllowedRel not working

[No One]

Hi, I have some code like this:

require_once __DIR__ . '/htmlpurifier/library/HTMLPurifier.auto.php';


$HTMLPurifierConfig = HTMLPurifier_Config::createDefault();
$HTMLPurifierConfig->set('HTML.Doctype', 'XHTML 1.1');
$HTMLPurifierConfig->set("Attr.AllowedRel", array('ugc noopener noreferrer' => true)); $purifier = new HTMLPurifier($HTMLPurifierConfig);

// Then I call:

echo $purifier->purify('<a href="https://htmlpurifier.org">HTML Purifier');

// And I get:

<a href="https://htmlpurifier.org">HTML Purifier</a>

// without the rel="ugc noopener noreferrer". Why is this?