protected function loadExtensions()
    {
        if ($this->extensionsLoaded) {
            return;
        }
        foreach ($this->extensions as $extension) {
            foreach ($extension->getTags() as $tag) {
                if (!$tag instanceof TagInterface) {
                    throw new Html2PdfException('The ExtensionInterface::getTags() method must return an array of TagInterface.');
                }
                $this->addTagObject($tag);
            }
        }

        $this->extensionsLoaded = true;
    }