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<!--#set var="revision" value="\$Id: sgml.html,v 1.16 2004-07-21 23:43:21 link Exp $"
--><!--#set var="date" value="\$Date: 2004-07-21 23:43:21 $"
--><!--#set var="title" value="Introduction To SGML for The W3C Markup Validation Service"
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<div class="doc"><a id="skip" name="skip"></a>
<h2>Introduction to SGML<br /> for the W3C Markup Validator</h2>
<div>
<h3 id="sgml">What is SGML?</h3>
<p>
SGML stands for Standard Generalized Markup Language. This is
actually a slight misnomer, since SGML is actually a
<em>meta-language</em> — that is, a language for writing markup
languages. HTML is a markup language written in SGML — an "SGML
application", to use the terminology.
</p>
<p>
You don't actually have to know much about SGML to use The Validator
successfully. If you're interested, though, I recommend TEI's
<a href="http://www.tei-c.org/P4X/SG.html">"A Gentle
Introduction to SGML"</a> as a good starting point. For in-depth
treatment of <acronym title="Standard Generalized Markup Language">SGML</acronym>
and <acronym title="HyperText Markup Language">HTML</acronym> I recommend
Martin Bryan's "<a href="http://www.is-thought.co.uk/book/home.htm">Web SGML and HTML 4.0 Explained</a>".
</p>
</div>
<div>
<h3 id="dtd">What is a DTD?</h3>
<p>
For our purposes, a DTD, or Document Type Definition, is simply a file
that defines the syntax of a <a href="#sgml">SGML</a>-based language.
The DTDs for
<a href="http://www.w3.org/MarkUp/html-spec/">HTML 2.0</a>
and <a href="http://www.w3.org/TR/REC-html32">HTML 3.2</a>
were written by the HTML Working Group of the
<a href="http://www.ietf.org/"><abbr title="Internet Engineering Task Force">IETF</abbr></a>,
in collaboration with the <a href="http://www.w3.org/"><abbr title="World Wide Web Consortium">W3C</abbr></a>.
From <a href="http://www.w3.org/TR/html4/">HTML 4.0</a> on (this includes
<a href="http://www.w3.org/TR/xhtml1/">XHTML</a>), the standards (both
prose and DTDs) have been written by the
<a href="http://www.w3.org/"><abbr title="World Wide Web Consortium">W3C</abbr></a>.
</p>
</div>
<div>
<h3 id="doctype">What is this <code>DOCTYPE</code> thing The Validator
keeps pestering me for?</h3>
<p>
A <code>DOCTYPE</code> is a <a href="#sgml">SGML</a> document type
declaration. Its purpose is to tell an SGML parser what
<a href="#dtd">DTD</a> it should use to parse the document. It appears
as the first line of the document, and has the form:
<code><!DOCTYPE html PUBLIC "quoted string"></code>
</p>
<p>
The quoted string is called a <dfn>public identifier</dfn>; it refers
to the desired DTD by a "well-known" name, usually defined by an
associated standard.
</p>
<p>
Most Web browsers don't actually use an SGML parser (in fact, none
that I'm aware of do), and so they don't need a <code>DOCTYPE</code>
declaration, and will ignore it if present. The Validator, however,
does use an SGML parser, and therefore needs a <code>DOCTYPE</code>
declaration. The Validator is more insistent on this point than
WebTechs was, which would insert a <code>DOCTYPE</code> on the fly
for you; The Validator requires that your <code>DOCTYPE</code> already
be in the document.
</p>
<p>
So now you're preparing to add a <code>DOCTYPE</code> to your document.
Be sure that the syntax is as described above, and that you use the
correct public identifier; otherwise, The Validator will use the wrong
DTD, or will be unable to find a DTD at all, and will produce a huge
list of absolutely meaningless errors.
</p>
<p>
The W3C QA Activity maintains a <a
href="http://www.w3.org/QA/2002/04/valid-dtd-list.html">List of
Valid Doctypes</a> that you can choose from, and the <acronym
title="Web Design Group">WDG</acronym> maintains a document on
"<a href="http://www.htmlhelp.com/tools/validator/doctype.html">Choosing
a DOCTYPE</a>".
</p>
<p class="warning">
<strong>WARNING:</strong> Some HTML editors will insert a
<code>DOCTYPE</code> declaration for you. Unfortunately, this
pre-inserted <code>DOCTYPE</code> will sometimes confuse
The Validator. This usually occurs when the inserted
<code>DOCTYPE</code> does not correspond to the generated HTML.
If your editor adds a <code>DOCTYPE</code> to your page, you may
need to correct it as described above before running your page through
The Validator.
</p>
</div>
</div>
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