code: def render_html(self, request, instance, context):
          
          render_template = self.get_render_template(request, instance, email_format='html')
          if not render_template:
              return str(u"{No HTML rendering defined for class '%s'}" % self.__class__.__name__)
  
          instance_context = self.get_context(request, instance, email_format='html', parent_context=context)
          instance_context['email_format'] = 'html'
  
          html = self.render_to_string(request, render_template, instance_context)
          if self.render_replace_context_fields:
              html = replace_fields(html, instance_context)  # pass safe-string
          return html
docs: Custom rendering function for HTML output