HTML5 な文書を DOMDocument::loadHTML 使って読み込ませると Warning が出る。 <?php $html = '<!doctype html><html><body><video loop><source src="test.mp4"></source></video></body></html>'; $dom = new \DOMDocument(); $dom->loadHTML( $html ); var_dump( $dom->getElementsByTagName( "video" )->length ); //Warning: DOMDocument::loadHTML(): Tag video invalid in Entity, line: 1 in /path/to/sample.php on line 4 //Warning: DOMD