1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
|
<!--#set var="title" value="Introduction To SGML for The W3C Markup Validation Service"
--><!--#set var="relroot" value="../"
--><!--#include virtual="../header.html" -->
<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> we 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>
<h4>Why add a DOCTYPE declaration?</h4>
<p>
The Validator uses an SGML parser, and a <code>DOCTYPE</code> declaration
is the most, if not the only way to know which markup language it should
validate documents against.
</p>
<p>
Note that most Web browsers don't actually use an SGML parser, many of them display
the documents differently based on the document's <code>DOCTYPE</code>
declaration, or lack thereof. This alone is a good reason to always
add a <code>DOCTYPE</code> declaration to Web documents.
</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>
<h4>How do I add a DOCTYPE declaration?</h4>
<p>
The W3C QA Activity maintains a <a
href="http://www.w3.org/QA/2002/04/valid-dtd-list.html">List of
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, sometimes
thia <code>DOCTYPE</code> does not correspond to the generated HTML,
which can sometimes confuse The Validator.
If your editor adds a <code>DOCTYPE</code> to your page, you may
need to correct it as described above before running your documents through
The Validator.
</p>
</div>
</div>
<!--#include virtual="../footer.html" -->
</body>
</html>
|
