Remove the width and height attribute from all WordPress images

functions.php

<?php
/**
 * Remove width and height from all the WordPress images.
 * 
 * @param $html
 *
 * @return mixed
 */
function remove_thumbnail_dimensions( $html ) {

	$html = preg_replace( '/(width|height)=\"\d*\"\s/', "", $html );

	return $html;
}
add_filter( 'post_thumbnail_html', 'remove_thumbnail_dimensions', 10 );
add_filter( 'the_content', 'remove_thumbnail_dimensions', 10 );
add_filter( 'image_send_to_editor', 'remove_thumbnail_dimensions', 10 );