A semi-random thought that occurred to me.
One marker of a well-designed markup language is that it looks to the future. This doesn’t mean it’s an amorphous blob of abstract indirections mapped to tags. It can (and arguably should) be concrete and solid, but designed in such a way that keeps bigger things in mind.
HTML and XHTML are, I suppose, canonical examples of this, giving birth to microformats and many other uses outside of a browser. -m