I joined mailing lists discussing the HTML spec and dove into the HTML 2.0 Internet Draft. The Internet Draft talked about SGML, so I dove into SGML.
The only book in the University library on SGML was Martin Bryan's SGML: An Author's Guide to the Standard Generalized Markup Language so that became my handbook.
I became fascinated with the SGML philosophy of separating presentation from semantic markup.
And quite apart from the Web, I found out about SGML being used by things like the Text Encoding Initiative (TEI) which was immediately interesting to me because of application to corpus linguistics and biblical studies.