Several Racket libraries are tagged "sxml," but I wonder which formats they use exactly.
As for SXML, I think it's pretty nice that the SXML standard actually has well-defined ways to use doctypes, namespaces, processing instructions, and comments. That makes it more capable of representing an actual XML document. The spec is only a few pages long, and half of that is preamble, so it would be pretty simple to implement it faithfully.
All in all, I think I'd like to write code in a style like sml.arc, but then normalize it to something like SXML.