During my holiday travels this year I discovered what may be the “missing tag” in Carnac, south-west Brittany. Here is photographic evidence that 7,000 years ago neolithic technologists were interested in structured markup.
Interestingly, the glyphs represented don’t seem to be present in the latest version of the Unicode standard. Did our ancestors know something we don’t?

Actually, I think they were using SGML, as evidenced by implicit close tag.
Dom, I guess you’re right. SGML or even GML maybe!
Or was the end tag at the other end of the woods?
I went all the way to the end of forest (EOF) and I couldn’t find a closing tag
Maybe their early ideas just weren’t well-formed enough?
We may never know. Only fragments survive