public function registerHTML($id, $html, $linear = 'yes', array $properties = []){
		$dom = $this->parser->parseFromString($html);
		if( $dom ){
			if( !isset($this->doms[$id]) ){
				$this->opf->addIdref($id.'.xhtml', $linear, $properties);
			}
			$this->doms[$id] = $dom;
			return $dom;
		}
		return false;
	}