docs: def progressive(image_field, alt_text=''):
      """
      Used as a Jinja2 filter, this function returns a safe HTML chunk.
  
      Usage (in the HTML template):
  
          {{ obj.image|progressive }}
  
      :param django.db.models.fields.files.ImageFieldFile image_field: image
      :param str alt_text: str
      :return: a safe HTML template ready to be rendered
      """
      if not isinstance(image_field, ImageFieldFile):
          raise ValueError('"image_field" argument must be an ImageField.')
  
      for engine in engines.all():
          if isinstance(engine, BaseEngine) and hasattr(engine, 'env'):
              env = engine.env
              if isinstance(env, Environment):
                  context = render_progressive_field(image_field, alt_text)
                  template = env.get_template(
                      'progressiveimagefield/render_field.html'
                  )
                  rendered = template.render(**context)
                  return Markup(rendered)
      return ''