Modify any specific element knowing its parent class via WP Rocket buffer [WordPress]

modify_specific_element_via_wp_rocket_buffer.php

//Add missing width and height to the specific img to avoid layout shift, knowing its parent element class
//Or modify anything at all via WP Rocket buffer
add_filter('rocket_buffer','sz_wprocket_modify_buffer', 10);
function sz_wprocket_modify_buffer ($buffer){
$dom = new DOMDocument();
$dom->loadHTML($buffer);
$xpath = new DOMXPath($dom);
	$expression = './/div[contains(concat(" ", normalize-space(@class), " "), " mypic ")]'; //tag and class here
	foreach ($xpath->evaluate($expression) as $parentelement) 	{
		foreach ($parentelement->getElementsByTagName('img') as $tag) { //targeting img inside the parent tag
               // $tag->setAttribute('szattribute', 'szvalue'); //or add any other attribute
			    $tag->setAttribute('width', '');
			    $tag->setAttribute('height', '');
            }
	}
	$buffer = $dom->saveHtml();	
	return $buffer;
}