What markup language is this? (ca. 1991)

I found this PDF on Xyvision Production Publisher (which is likely what was used). Note under FinalPages it lists HTML as an output format.

If you could somehow get a copy of some version of this software running, you may be able to get it to spit out some HTML. That may or may not be harder than reverse-engineering the document markup. There's a bit more info on HTML exports at the very bottom of this page.

It is possible that the product used to generate your files has evolved into Portalyx SDL XPP. It might be worth contacting them.

See XPP Personal Edition

Identified via a dead link on a Wikipedia page which mentioned XyVision XPP.