code: protected void renderId(
        FacesContext context,
        UIComponent  component) throws IOException
      {
        if (shouldRenderId(context, component))
        {
          String clientId = getClientId(context, component);
          context.getResponseWriter().writeAttribute(HTML.ID_ATTR, clientId, JSFAttr.ID_ATTR);
        }
      }
docs: Renders the client ID as an "id".