protected function getLinkCssHref(array $tag_frag, $test_for_css = true)
    {
        if ($test_for_css && !$this->isLinkTagFragCss($tag_frag)) {
            return ''; // This tag does not contain CSS.
        }
        if (preg_match('/\shref\s*\=\s*(["\'])(?P<value>.+?)\\1/ui', $tag_frag['link_self_closing_tag'], $_m)) {
            return trim($this->nUrlAmps($_m['value']));
        } // unset($_m); // Just a little housekeeping.

        return ''; // Unable to find an `href` attribute value.
    }