I guess I’m unclear what benefit this has over a more generic attribute extension that can be used in many places, such as that used by Pandoc or kramdown, like adding {#identifier .class .class key=value key=value}.
If the number of unescaped
#characters in the opening sequence is matched exactly by the number of unescaped#characters in a possible closing sequence with a space on both sides, the characters after this closing sequence form an info string and are not used verbatim for output.
I feel like this style is going to get complicated when trying to extend this functionality to other elements.
Intuitively, I feel like having a consistent attribute syntax, as talked about here would be a better overall solution.
In fact, I wish we would just either choose the Pandoc or kramdown style, tweak if necessary, and get it done 
It’s really something that’s needed…