Since the 1990s, the web has been a publishing place for human-readable documents.

Documents published on the web are in HTML. HTML has a little bit of structure, for example, “here is a paragraph” or “emphasize this word.”

Then you stir in some CSS, which adds some pretty decorations to the structure, saying things like: make those paragraphs have tiny gray sans-serif text! And then people think you are hip. Unless they are older, and they can’t read your tiny gray words, so they give up on you.

That’s “structure,” as far as it goes, on the web.

Imagine, for example, that you mention a book on the web.

Goodnight Moon

by Margaret Wise Brown

Illustrated by Clement Hurd

Harper & Brothers, 1947

ISBN 0-06-443017-0

There’s not much structure there. A naive computer program reading this web page might not realize I was even mentioning a book.…