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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>W3C HTML Validator SGML Intro</title>
<link rev="made" href="mailto:gerald@w3.org" />
<link rel="stylesheet" type="text/css" href="/base.css" />
<meta name="keywords" content="HTML, Hypertext Markup Language, Validation, W3C HTML Validation Service" />
<meta name="description" content="W3C's easy-to-use HTML validation service, based on an SGML parser." />
<meta name="revision" content="$Id: sgml.html,v 1.1 2001-02-23 09:06:49 link Exp $" />
<meta name="modified" content="$Date: 2001-02-23 09:06:49 $" />
</head>
<body bgcolor="#FFFFFF" text="#000000" link="#0000ee" vlink="#551a8b">
<h1><a href="http://www.w3.org/"><img align="left"
src="http://www.w3.org/Icons/WWW/w3c_home" height="48" border="0"
alt="W3C" /></a> HTML Validator SGML Intro</h1>
<p align="right" class="navbar">
<a href="about.html">About this service</a> |
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<a href="source/">Source code</a> |
<a href="feedback.html">Feedback</a><br clear="right"/>
<a href="file-upload.html">Upload files</a> |
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<a href="checklink">Link checker</a><br clear="right"/>
<a href="http://www.w3.org/People/Raggett/tidy/">HTML Tidy</a> |
<a href="http://www.w3.org/MarkUp/">HTML home</a> |
<a href="http://www.w3.org/TR/html401/">HTML 4.01</a> |
<a href="http://www.w3.org/TR/xhtml1/">XHTML 1.0</a>
<br clear="all" />
</p>
<div>
<h2 id="sgml">What is SGML?</h2>
<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://etext.virginia.edu/tei-tocs1.html">"A Gentle
Introduction to SGML"</a> as a good starting point. An additional SGML
resource can be found on
<a href="http://www.oasis-open.org/cover/sgml-xml.html">SIL's SGML
Web Page</a>.
</p>
</div>
<div>
<h2 id="dtd">What is a DTD?</h2>
<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 DTD's for
<a href="http://w3.org/MarkUp/html-spec/">HTML 2.0</a>
and <a href="http://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://w3.org"><abbr title="World Wide Web Consortium">W3C</abbr></a>.
From <a href="http://w3.org/TR/html4">HTML 4.0</a> on (this includes
<a href="http://w3.org/TR/xhtml1">XHTML</a>), the standards (both
prose and DTDs) have been written by the
<a href="http://w3.org"><abbr title="World Wide Web Consortium">W3C</abbr></a>.
</p>
</div>
<div>
<h2 id="doctype">What is this <code>DOCTYPE</code> thing The Validator
keeps pestering me for?</h2>
<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. The Validator's
<a href="catalog">public identifier catalog</a> lists all the public
identifiers The Validator recognizes for various types of HTML; of
those, the following public identifiers are most likely to be widely
recognized:
</p>
<dl>
<dt><code>For HTML 2.0...</code></dt><dd>...use "-//IETF//DTD HTML 2.0//EN"</dd>
<dt><code>For HTML 4.0...</code></dt><dd>...use "-//W3C//DTD HTML 4.0//EN"</dd>
<dt><code>For HTML 4.0 Transitional...</code></dt><dd>...use "-//W3C//DTD HTML 4.0 Transitional//EN"</dd>
<dt><code>For HTML 4.0 Frameset...</code></dt><dd>...use "-//W3C//DTD HTML 4.0 Framset//EN"</dd>
</dl>
<p>Note that the string must appear exactly as shown, including case.</p>
<p class="warning">
<strong>WARNING:</strong></a> 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>
</body>
</html>
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