Add max_width to image attributes in RSS feed

function chris_coyiers_super_awesome_feed_image_magic( $content ) {
  $doc = new DOMDocument();
  $doc->LoadHTML( $content );
  $images = $doc->getElementsByTagName('img');
  
  foreach ($images as $image) {
    // Set the new attribute.
    $image->setAttribute( 'style', 'max-width:480px;' );
  }
  
  return $doc->saveHTML();
}
add_filter( 'the_content_feed', 'chris_coyiers_super_awesome_feed_image_magic' );