How to add a width attribute to a responsive image with srcset?
This is not going to be easy due to Drupal's way of rendering images, you should not touch this unless you know how to write code. The code below is untested, but should give you a direction.

function THEMENAME_preprocess_responsive_image_formatter(array &amp;$variables) {
  //this is the hook where you do get access to the image file entity
  //don't use 'width' as attribute name, it might get unset by Drupal in the following hook
  $variables['responsive_image']['#attributes']['data-original-image-width'] = $variables['item']-&gt;width;
}

function THEMENAME_preprocess_responsive_image(array &amp;$variables) {
  //this is the preparing the rendering, here you can set the width attribute again
  $variables['#attributes']['width'] = $variables['img_element']['#attributes']['data-original-image-width'];
}


This is code just an outline, and most likely will only work if responsive images module is configured for an single img[srcset] and will not work with &lt;picture&gt; tags.

The hooks are quite good documented though, check the API docs for more information

hook_preprocess_responsive_image_formatter

hook_preprocess_responsive_image