rel tag leaves W3C Validator error

I had been using rel=”totop” to smooth scroll and it works great, but when I run it through W3C Validator I get the following error:

Bad value totop for attribute rel on element a: The string totop is not a registered keyword

It seems I need to replace rel=”totop” with data-lightbox=”true” for it to validate, but in doing so I can’t connect it to this plugin.

Can anyone help?

https://wordpress.org/plugins/page-scroll-to-id/