protected static function callback_sanitize_css_iri($match) {
		$str = static::sanitize_attribute_value($match[1]);

		// Strip quotes.
		$str = ltrim($str, "'\"");
		$str = rtrim($str, "'\"");

		$str = static::sanitize_iri_value($str);

		if (strlen($str)) {
			return "url('$str')";
		}

		return 'none';
	}