Validation - a history.

I'm curious about the history of validation.
DTD's through to DSDL, is there any logical story/development?
Was it always present in SGML?
Did DTD's grow up with SGML or were they added?

Is it decaying with more data (compared to document) usage?



regards

-- 
Dave Pawson
XSLT XSL-FO FAQ.
http://www.dpawson.co.uk