Hi @svenkreiss and thank you for html5validator! 😃

To add a bit more background to @ThibautGery's PR: we use this package in the French government (@SGMAP) to validate public-facing websites such as https://api.gouv.fr and https://beta.gouv.fr.
In order to add external links in a safe way (betagouv/api.gouv.fr#150), we wanted to use rel="noopener" on a a, and had the surprise to discover it was not considered valid, even though the WhatWG spec mentions it as standard.

We would like to keep on using html5validator, but this is currently blocking our delivery. We hope this changeset can fix this issue and allow us to keep working with an updated version of html5validator :)

Thanks a lot! This is now released in v0.2.3 and uploaded to pypi.

Thank you very much for the quick update ! Our build is fixed !