public function addCustomElement($element)
    {
        if (($colon = strpos($element, ':')) !== false) {
            $namespace = substr($element, 0, $colon);
            if (!$this->hasNamespace($namespace)) {
                throw new \InvalidArgumentException(sprintf('Unregistered namespace: %s', $namespace));
            }
        }

        $this->customElements[] = $element;

        return $this;
    }