new_contents: from branca.element import Figure, Element, JavascriptLink, CssLink
  
  
  class JSCSSMixin(Element):
      """Render links to external Javascript and CSS resources."""
  
      default_js = []
      default_css = []
  
      def render(self, **kwargs):
          figure = self.get_root()
          assert isinstance(figure, Figure), ('You cannot render this Element '
                                              'if it is not in a Figure.')
  
          for name, url in self.default_js:
              figure.header.add_child(JavascriptLink(url), name=name)
  
          for name, url in self.default_css:
              figure.header.add_child(CssLink(url), name=name)
  
          super().render(**kwargs)
  
subject: Add mixin to render JS and CSS links