Surely you mean S-expressions. They're great, but not as a markup language.

What I show here is in fact even simpler and more flexible than S-exps[0].

[0] For some details and polemic see this thread: https://news.ycombinator.com/item?id=33334789 | TL;DR: Jevko is well-defined, basically just unicode text + escapeable brackets for making trees; it doesn't treat whitespace as a separator/atmosphere (particularly important in markup); it takes advantage of natural name-value pairing tendencies (like tag-children); and it's closed under concatenation by design