I have been reading again a few things this week:
Netscape provided extensions to HTML to please people who are used to desktop publishing software and expect similar capabilities from a browser.
Then a bit further.
These recommendations are given with the goal of producing documents that look as nice as possible in Netscape, while retaining their integrity in other browsers. Besides the obvious question of how other browsers will display the elements in question, how these elements effect the process of creating documents needs to be considered: i.e., do they make it easy to create documents that will do their job in other browsers, or do they encourage authors to create documents that will loose their headers in other documents.
If the goal is to create documents that won't have to be modified to be properly processed by future tools, then the answer is easy: don't use any of the NHTML extensions, and only use the most stable of the HTML3 features.
Where does it come from? From the past, exactly from 1995.