In Jupyter, now it's always right aligned!

It's one thing enforce right-alignment for consistency reasons as described in /pull/2534. It's another to stop supporting features in Markdown as described by the link in the Help bar, currently pointing to:

https://help.github.com/articles/getting-started-with-writing-and-formatting-on-github/
https://help.github.com/articles/organizing-information-with-tables/#formatting-content-within-your-table

Furthermore, the user gets no messaging to let them know that they're using unsupported features, i.e. the :---, :---:, and ---: tokens to describe justification.