[Unofficial mirror copy from: http://www.textuality.com/sgml-erb/dd-1996-0002.html, November 5, 1996 ]

Features and Rules of ISO 8879

A Summary for Use

In Discussions of the W3C SGML Working Group

And Editorial Review Board

Table of Contents

• 1 Introduction, Scope, Field of Application, References, Definitions, and Notation (Clauses 0-5)
• 2 Entity Structure (Clause 6)
• 3 Element Structure (Clause 7)
• 4 Processing Instruction (Clause 8)
• 5 Common Constructs (Clause 9)
• 6 Markup Declarations: General (Clause 10)
• 7 Markup Declarations: Document Type Definition (Clause 11)
• 8 SGML Declaration (Clause 13)

This document summarizes some of the salient features of ISO 8879 which may or may not require revision in XML. The `features' included here are not limited to those called features by the standard; they include other characteristics and rules as well.

In virtually all cases, the inclusion of an item in these lists may be interpreted as a suggestion that the W3C SGML Working Group and Editorial Review Board will need to decide explicitly whether XML should (a) work exactly the same way as SGML, (b) work in only one of the ways allowed by SGML, (c) provide the same functionality in some other way, or (d) suppress the feature / functionality in question. That is, after each statement, the question "Should this be true in XML as well?" may be understood implicitly, even if not explicit.

In some cases, an entry is present only as a reminder of some SGML construct which the compiler or others have found hard to follow or obscurely expressed in ISO 8879, and which should be described more clearly in XML documentation.

In addition to mentioning some SGML features and rules, the lists below indicate whether those features or rules are omitted or modified by various proposals for subsets or simplifications of SGML. The proposals thus summarized are:

• Bas (Basic SGML), as defined in ISO 8879, clause 15.1.1. A basic SGML document is one which uses only:
  • the reference concrete syntax
  • the reference capacity set
  • only the features shorttag and omittag
• LA (Lexical Analyzer for HTML), as defined in Dan Connolly, "A Lexical Analyzer for HTML and Basic SGML: W3C Working Draft 15-Jun-96" at http://www.w3.org/pub/WWW/TR/WD-sgml-lex. As the name of the document suggests, LA is a slight simplification of Basic SGML; in cases of doubt, therefore, entries for LA have been made to agree with those for Bas. In some cases, however, LA prohibits constructs which are legal in Bas. In others, the LA document divides responsibility for SGML conformance between the LA processor and the client software; in these cases, the entry for LA describes what the LA software proper does, with a note indicating what work is left to the client.
• MGML (Minimal Generalized Markup Language), as defined in Tim Bray, "MGML - an SGML Application for Describing Document Markup Languages", unpublished draft paper for SGML '96 ( http://www.textuality.com/mgml/index.html) and attachments, especially the `reference DSD' (document structure definition) as of 18 August 1996. Entries have been corrected by Tim Bray, and in some cases reflect plans for later revision of the proposal.
• Min (Minimal SGML), as defined in ISO 8879, clause 15.1.2. A minimal SGML document is defined as one which uses:
  • the core concrete syntax (i.e. the reference concrete syntax without any short reference delimiters)
  • the reference capacity set
  • no features
• NSGML (Normalised SGML), as defined in Henry Thompson, David McKelvie, and Steve Finch, "The Normalised SGML Library (NSL)", NSL Version 1.4.4, Documentation version Fri Aug 2 14:13:40 BST 1996 ( http://www.ltg.ed.ac.uk/corpora/nsldoc/nsldoc.html), in particular the section "Definition of NSGML". Those present have been reviewed by Henry Thompson, and in one case (conref) reflect plans for the next release of the NSGML Library.
• PSGML (Poor Folks SGML), as defined in C. M. Sperberg-McQueen, "PSGML: Poor-Folks SGML: A Subset of SGML for Use in Distributed Applications", Document UIC CC DB92-10, 8 October 1992 ( http://www.uic.edu/~cmsmcq/uic/db92-10.tei or http://www.uic.edu/~cmsmcq/uic/db92-10.html)
• SL (SGML Lite), as defined in Bert Bos, "`SGML-Lite' - an easy to parse subset of SGML", 4 July 1995 ( http://grid.let.rug.nl/~bert/Stylesheets/SGML-Lite.html). The SGML Lite document prescribes the core concrete syntax, but gives a full syntax declaration for the reference concrete syntax, which includes the standard short references; the entries below reflect the core syntax, not the reference syntax. The omittag feature is not discussed; the entries for SL reflect the apparent intention of setting OMITTAG NO. In cases of doubt, the entries for SL agree with those of Bas.
• SO (SGML Online), as defined in Eliot Kimber, "SGML Revision, Proposal for Minimal SGML Feature Set" (unfinished, unpublished draft, distributed privately via email on 1996-06-03, now accessible at http://www.textuality.com/sgml-erb/kimber/index.html). SO is formulated largely as a proposal to add new optional features to SGML, to enable applications to avoid supporting certain constructs which are not now optional. The entries for SO in the lists below describe the constructs which would be kept, modified, or suppressed by applications making use of the proposed new options. In cases where the SO document is silent, its entries have been made to agree with those of Min, on the theory that the point of SO is to make possible the construction of light-weight SGML parsers, and that SO implementors are likely to make use of all the simplifications already offered by Min.
• TEI (TEI Interchange Format), as defined in Association for Computers and the Humanities (ACH), Association for Computational Linguistics (ACL), and Association for Literary and Linguistic Computing (ALLC), Guidelines for Electronic Text Encoding and Interchange (TEI P3), ed. C. M. Sperberg-McQueen and Lou Burnard (Chicago, Oxford: Text Encoding Initiative, 1994), especially section 28.1.3 "TEI Interchange Format" and chapter 39 "Formal Grammar for the TEI-Interchange-Format Subset of SGML". The TEI Guidelines are available on the World-Wide Web at http://etext.virginia.edu/TEI.html and at http://dynaweb.ebt.com/usrbooks/teip3/1.toc. The sections relevant in this context are collected in a single file (for easier printing) in http://www-tei.uic.edu/orgs/tei/ml/tif.html. In preparing this summary, I have also consulted Association for Computers and the Humanities (ACH), Association for Computational Linguistics (ACL), and Association for Literary and Linguistic Computing (ALLC), Guidelines for the Encoding and Interchange of Machine-Readable Texts (TEI P1), ed. C. M. Sperberg-McQueen and Lou Burnard (Chicago, Oxford, 1990), section 2.2.

Except as noted, entries have not been reviewed by the authors of the proposals, and so may be in error. Some of these schemes also include proposals that do not fit readily into the format of this list, and are not described here. For full and authoritative information, the documenation for each proposal should be consulted.

Other schemes may be added if we learn of them and they seem to be of interest.

In summarizing the schemes, the following abbreviations are used:

• sic (`thus'). The schemes indicated agree with ISO 8879 on this point.
• om. (`omitted by'). The schemes indicated omit or suppress the construct, concept, or syntax described.
• mod. (`modified by'). The schemes indicated modify or restrict the construct, concept, or syntax in some way (which may be described briefly -- for full information, however, consult the relevant documentation).
• dna (`does not apply to'). The rule does not apply to the schemes indicated, because it applies to a situation which cannot arise in those schemes, normally because they have suppressed one of the constructs involved. In some cases, there is a fine line between om. and dna; the distinction should not be made to bear too much weight.
• sil. (`passed over in silence by'). The schemes indicated don't address the issue one way or another, and the compiler is not confident that this silence indicates agreement with 8879.

If no note is present, it normally means all the schemes collated to date agree with 8879. In isolated cases, it may mean the compiler just skipped over the item due to inattention, boredom, or a conviction that a note was not necessary. Notes are not normally given for quantity definitions, because few of the schemes collated propose changes to the default or minimum quantities; most either leave the quantities as they are, or propose ignoring or dropping them entirely.

For example, consider the following entry:

• 7.5 defines three forms of minimized end-tags:
  • empty end-tag
  • unclosed end-tag
  • null end-tag
  These are allowed if and only if SHORTTAG YES is specified.
  
  [Sic Bas; om. MGML, Min; mod. PSGML. PSGML allows empty end-tags, despite having SHORTTAG NO, but not null or unclosed end-tags.]

This means (1) that minimized end-tags are defined in clause 7.5, and (2) that in basic SGML, all three forms of minimized end-tags are legal and may be used as described in 8879, while none may be used in MGML or minimal SGML, and only one in PSGML. When more than one item is derived from a clause, a letter is attached to the clause number to keep the items distinct; it is not part of the formal clause numbering of ISO 8879.