The HTML Validation HOWTO
  Keith M. Corbett, kmc@specialform.com
  v0.2, 29 October 1995

  This document explains how to use the nsgmls parser to validate HTML
  documents for conformance with the HTML 2.0 document type definition,
  or "DTD".  This DTD is the most commonly accepted SGML based defini-
  tion of HTML, and thus defines a subset of current practice in HTML
  markup that is likely to be portable to a wide number of HTML users
  agents (browsers).
  ______________________________________________________________________

  Table of Contents:

  1.      Introduction

  1.1.    Costs and benefits

  1.2.    Getting started

  2.      Tools

  2.1.    The

  2.2.    The

  2.3.    Download the HTML specification materials

  3.      Parsing an HTML document

  3.1.    Parser input

  3.2.    Parser output

  3.3.    Parser messages

  3.4.    Return status

  4.      Resources
  ______________________________________________________________________

  1.  Introduction

  This is a guide to using the nsgmls parser to validate and process
  HTML documents.

  1.1.  Costs and benefits

  Using the full features of SGML markup will enrich your HTML
  documents.  However, validating your documents to the HTML DTD has
  certain cost / benefit tradeoffs, basically because you are dealing
  with a more circumscribed dialect of HTML than is currently in vogue.
  The "official" HTML rules for enforcing document structure, and the
  SGML rules for data content markup, are more restrictive than current
  practice on the Web.

  The main issue you must consider is that valid HTML is restricted to a
  standard set of element tags.

  There isn't an accepted DTD that accurately reflects "browser HTML" as
  understood by many client browser programs.  For the most part, the
  HTML 2.0 DTD reflects tags and attributes that were commonly in use on
  the Web around June 1994.  Various efforts to define a more advanced
  HTML+ or HTML 3.0 DTD have gotten somewhat bogged down.  And none of
  the DTDs in circulation will recognize all of the tags that have been
  popularized recently by browser vendors such as Netscape and
  Microsoft.

  1.2.  Getting started

  Contrary to popular opinion, working with SGML does not have to cost a
  lot of time and money.  It is possible to build a robust development
  environment consisting entirely of software that is freely available
  on a wide range of platforms, including Linux, DOS, and most Unix
  workstations.  Thanks to a few very dedicated folks, all the tools you
  need to work with SGML have been made publicly available on the
  Internet.

  Setting up your environment (the parser and supporting program
  libraries) takes a bit of work but not nearly as much as one might
  think.

  You may also want to peruse an introductory SGML text such as "SGML:
  An Author's Guide to the Standard Generalized Markup Language" by
  Martin bryan, or "Practical SGML" by Eric van Herwijnen.

  2.  Tools

  2.1.  The HTML Check toolkit package

  If you want a completely self-installing / canned package, check out
  the HalSoft HTML Check Toolkit at URL: http://www.halsoft.com/html-
  tk/index.html

  The only disadvantage of using the HalSoft kit is that it uses the
  older sgmls parser, which produces error messages that are sometimes
  (even) more cryptic than those from nsgmls.

  I've used nsgmls on Linux and Windows (3.x and NT); it is supposed to
  work on many other platforms as well.

  2.2.  The nsgmls parser

  James Clark has built a software kit called sp which includes the
  validating SGML parser, nsgmls.  (This is the successor to the sgmls
  parser which has long been considered the reference parser.)

  For information on the sp kit, see URL: http://www.jclark.com/sp.html

  You can download the kit directly from: ftp://ftp.jclark.com/pub/sp/

  You may be able to pick up nsgmls executable files for your platform.
  Or, download the source kit and follow the directions in the README
  file for running make.

  Consider creating a high level public directory that will contain
  SGML-related files.  For example, on my Linux PC I have various SGML
  related directories including:

  /usr/sgml/bin

  /usr/sgml/html

  /usr/sgml/sgmls

  /usr/sgml/sp

  2.3.  Download the HTML specification materials

  The draft standard for HTML 2.0 includes SGML definition files you
  need to run the parser, namely the DTD (Document Type Definition),
  SGML Declaration, and entity catalog.  To obtain the HTML 2.0 public
  text, see URL:

  http://www.w3.org/hypertext/WWW/MarkUp/html-spec/

  Download and install the following files:

  DTD html*.dtd

  SGML declaration html.decl

  Entity catalog catalog

  You can add two entries to the HTML entity catalog for ease of use
  with nsgmls:

       ______________________________________________________________________
               -- catalog: SGML Open style entity catalog for HTML --
               -- $Id: catalog,v 1.2 1994/11/30 23:45:18 connolly Exp $ --
        :
        :
               -- Additions for ease of use with nsgmls --
       SGMLDECL        "html.decl"
       DOCTYPE HTML    "html.dtd"
       ______________________________________________________________________

  Alternatively, you can create a second catalog containing these
  entries; you will have to pass this catalog to nsgmls as an argument
  with the -m switch.

  3.  Parsing an HTML document

  Following is a "cookbook" for validating a single document.  Simply
  invoke the nsgmls parser and pass it the pathnames of the HTML catalog
  file(s) and the document:

       % nsgmls -s -m /usr/sgml/html/catalog <test.html

  The -s switch suppresses the parser's output; see below.

  3.1.  Parser input

  Your document must conform to SGML, which means, among other things,
  that the document type must be declared at the beginning of the input.
  (You can fudge this by prepending the information to the document
  instance on the nsgmls command line.)

  Here's a simple HTML document that can be parsed correctly using the
  scheme I've outlined:

       ______________________________________________________________________
       <!doctype html public "-//IETF//DTD HTML 2.0//EN">
       <html>
       <head>
       <title>Simple HTML document.</title>
       </head>
       <body>
       <h1>Test document</h1>
       <p>This is a test document.</p>
       </body>
       </html>
       ______________________________________________________________________

  3.2.  Parser output

  The standard output of nsgmls is a digested form of the SGML input
  that processing systems can use as a lexer for navigating the
  structure of the document.  For the purpose of validation, you can
  throw the standard output away and rely on the error output.

  If you do want the full output, omit the -s switch and pipe standard
  output to a file:

       % nsgmls -m /usr/sgml/html/catalog <test.html >test.out

  3.3.  Parser messages

  Error and warning messages from nsgmls can be very cryptic.  And you
  may see very many errors from illegal markup.

  To pipe messages to a file, use the -f switch:

       % nsgmls -s -m /usr/sgml/html/catalog -f test.err <test.html

  3.4.  Return status

  The parser indicates whether the input document conforms to the HTML
  DTD in two ways:

  Return code - the parser returns a 0 exit status on success, non-zero
  otherwise.

  Output - if the document conforms to the DTD, the last line of
  standard output will consist of a single C character.

  4.  Resources

  The HalSoft HTML Check Toolkit is at URL: http://www.halsoft.com/html-
  tk/index.html

  James Clark's page on sp is at URL: http://www.jclark.com/sp.html

  The W3C page on the HTML specification is at URL:
  http://www.w3.org/hypertext/WWW/MarkUp/html-spec/

  Feel free to contact me via email: kmc@specialform.com.