XMPP shows pretty well that XML can do things that cannot be done easily without it. XMPP wouldn't work nearly as well with JSON. Namespaces are a super power.

IMO, the best thing about YAML is the referencing. It's super easy to reuse an object multiple times. Gives that same kind of parten child struct ability that programming languages have. Sure XML can do it, but it's not in every parser. cough python built in parser cough But then YAML is also not a built in parser and doing DOM in things other than XML feels odd.

Skimming through the post, the code snippet about halfway through picked my attention. Been a while since I studied site development, but that snippet looks awfully like HTML. Are it and XML related?

ASN.1 crying in the corner.

Preaching the choir I like to sing in.

I didn't know the link to S-Expressions, ty.