[ 14 October 1997
The Linux HTML Validation mini-HOWTO is not being maintained by
the author any more. If you are interested in maintaining the
HTML Validation mini-HOWTO, please get in touch with me at
<gregh@sunsite.unc.edu>. ]
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.