@mofosyne While I like your idea of having only a URL/identifier in the (), actually Common Mark defines images already as and that’s kind of why I worte that the part between () “is a string to be interpreted by the custom directive that generally is not going to be directly visible”. But we could of course do it “better” for directives than what happened to the image syntax, but then again that’s kind of restricting directives more than really necessary.
I think {.myClass width=40 height=50} is great and is in fact already proposed in the consistent attribute syntax thread. About the comma-separated variant: personally, I don’t like there being too many ways to write the same thing and think this is already “semi-json” enough.
About container block directives: for fenced code blocks, the standard currently says to use:
```ruby
and not:
```.ruby
So I think that’s consistent with my proposal. I agree that the spacing probably should be allowed there as you suggested and that also the [] might be optional there.
Finally, the @ and @@@ is a rather arbitrary choice. But it should be some special character that is easily distinguishable from ! (to make it easy to see that it’s not an image) and $ is often already used to mark up mathematics.