responsive_image_preprocess_responsive_image() and responsive-image.html.twig never render width and height variables despite claiming otherwise

Problem/Motivation

From responsive_image_preprocess_responsive_image():

  // Make sure that width and height are proper values
  // If they exists we'll output them
  // @see http://www.w3.org/community/respimg/2012/06/18/florians-compromise/
  if (isset($variables['width']) && empty($variables['width'])) {
    unset($variables['width']);
    unset($variables['height']);
  }
  elseif (isset($variables['height']) && empty($variables['height'])) {
    unset($variables['width']);
    unset($variables['height']);
  }

… but those variables never make it into actual HTML. They are not merged into some general attributes array. Nor are they rendered explicitly in the responsive-image.html.twig template.

Proposed resolution

Either support those variables, or remove them. The current documentation is very vague.

Remaining tasks

TBD

User interface changes

None.

API changes

TBD

Data model changes

TBD