I agree that we need extensible lightweight markup languages, but I'm not sure how the OP distinguishes between a grammar and a constraint. And if by "language

I agree that we need extensible lightweight markup languages, but I'm not sure how the OP distinguishes between a grammar and a constraint. And if by "language" he didn't mean "grammar," then what did he mean? An example would have gone a long way. My experience---and I've seen others comment to this effect---is that James Clark's nXml mode for emacs is as good as it gets for authoring in XML. If you already know the commands for structured editing, you have a great headstart. Also from James Clark is compact Relax NG. It's basically like regex for the DOM, and I find it pleasant to use. Until you hit something that it can't specify. But if you really must have a schema, and you have control over it, it can be worth making your schemas RelaxNG-able. And anyway, even if you're using markdown or anything else, using it without a "really sophistocated editor" is (IMHO) "next to impossible" (edit: I mean, just as painful as anything else). But yes, I've been ruminating over what I would use to replace Org Mode for literate programming, where I don't want all of its features (or an Emacs dependency), yet I do want to create ad-hoc constructs with arbitrary transforms during export. I haven't tried it, but it looks like Quaint might be of interest in that area. (http://breuleux.github.io/quaint/ http://breuleux.github.io/quaint/)