meaning and rationale of reference to SGML rules found in HTML 4 section 3.2.1

I am trying to understand the meaning and rationale of a sentence
added to the HTML 4 standard in section 3.2.1. The sentence first
appeared in the edition REC-html40-971218:

"Please consult the SGML standard for information about rules
governing elements (e.g., they must be properly nested, an end tag
closes all omitted start tags up to the matching start tag (section
7.5.1), etc.)."
-- http://www.w3.org/TR/REC-html40-971218/intro/sgmltut.html#h-3.2.1

I imagine there was some discussion of that addition, but I cannot
find any public mailing lists which have a record of it. Other than
public mailing lists, I really don't know where to start looking. If
someone would be so kind as to point me in the right direction ...

Cheers,

Edward McGuire
Fort Worth, Texas