Question

I have well formed HTML files. To turn them into SGML do I just switch the extension or is there more to do?

Answer 1

It’s going to depend on what version of HTML. From SGML:

While HTML was developed partially independently and in parallel with SGML, its creator Tim Berners-Lee, intended it to be an application of SGML. The design of HTML (Hyper Text Markup Language) was therefore inspired by SGML tagging, but, since no clear expansion and parsing guidelines were established, most actual HTML documents are not valid SGML documents. Later, HTML was reformulated (version 2.0) to be more of an SGML application, however, the HTML markup language has many legacy- and exception- handling features that differ from SGML’s requirements. HTML 4 is an SGML application that fully conforms to ISO 8879 – SGML.

The charter for the recently revived World Wide Web Consortium HTML Working Group says, "the Group will not assume that an SGML parser is used for ‘classic HTML’". Although HTML syntax closely resembles SGML syntax with the default reference concrete syntax, HTML5 abandons any attempt to define HTML as an SGML application, explicitly defining its own parsing rules which more closely match existing implementations and documents. (It does, however, define an alternative XML-based XHTML serialization, which does conform to SGML (WWW).)

So it looks like you probably already have SGML if you have well-formed HTML 4 or XHTML. Anything earlier (unlikely) or later (HTML 5) and you may have to make some changes to the document itself.