Warn on unknown attributes

[meithecatte]

For example:

> {pause exec-at-pause}
> ```slip-script
> ...
> ```

This will not work, because exec-at-unpause got typo'ed as exec-at-pause.

Of course, some attributes might still be used by user code. I'd suggest adopting the data-* namespace for this usecase.

[panglesd]

Yes, that's a good point. I can feel the unpleasant debugging experience.

Some html element interpret non-data-* attributes (eg width and height for images) so I wonder what is the best way to warn enough but not too much.

[meithecatte]

There's gotta be an easily available list of standard attributes somewhere. I think including any of those in a presentation is also fair game.