The HTML Validation HOWTO Keith M. Corbett, 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: 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: You can download the kit directly from: 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: 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 Simple HTML document.

Test document

This is a test document.

______________________________________________________________________ 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.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