Iterative post-order traversal of the DOM to remove empty nodes.

remove_empty_descendants.php

<?php
  /**
	 * Post-order traversal of the dom starting at given node. Remove any
	 * nodes that have no children and no attributes. Nodes named in the
	 * AMP_Rule_Spec::node_types_to_allow_empty array will not be removed.
	 */
	private function remove_empty_descendants( $node ) {
		$stack = array();
		$next_traversal = 'child';
		$nodes_to_remove = array();
		$starting_node = $node;
		while ( $node ) {
			if ( 'child' == $next_traversal ) {
				if ( $node->firstChild ) {
					$node = $node->firstChild;
					$next_traversal = 'child';
				} else {
					if ( XML_TEXT_NODE != $node->nodeType ) {
						if ( ( ! $node->hasAttributes() ) && 
							( ! in_array( $node->nodeName, AMP_Rule_Spec::node_types_to_allow_empty ) ) ) {
							$nodes_to_remove[] = $node;
						}
					}
					$next_traversal = 'sibling';
				}
			} elseif ( 'sibling' == $next_traversal ) {
				if ( $node->nextSibling ) {
					$node = $node->nextSibling;
					$next_traversal = 'child';
				} else {
					$node = $node->parentNode;
					if ( $node && $node->isSameNode( $starting_node ) ) {
						$node = null;
					}
					$next_traversal = 'sibling';
				}
			}
		}

		foreach( $nodes_to_remove as $node ) {
			if ( $node->parentNode ) {
				$node->parentNode->removeChild( $node );
			}
		}
	}
?>