Document Type Definition
A DTD is a "Document Type Definition" which specifies the syntax
(grammatical structure) of a web page in
SGML,
the Standard Generalised Markup Language
- of which HTML is an application.
SGML
(Standard Generalized Markup Language)
is an international standard for the definition of device-independent,
system-independent methods of representing texts in electronic form.
More precisely, SGML is a metalanguage, i.e. a method of
formally describing a language, in this case, a markup language.
HTML is a markup language written in SGML - an "SGML application".
HTML is a specific application or document type of the meta-language
SGML.
HTML is specified in an SGML document type definition or 'DTD'.
An HTML DTD describes in precise, computer-readable language the
allowed syntax and grammar of HTML markup.
The formal specification is represented by "DTDs".
A DTD, or Document Type Definition file is used by
SGML
applications, e.g. HTML, to specify rules that apply
to the markup of documents of a particular type,
including a set of element and entity declarations.
There are various versions of HTML, each described by a unique DTD.
Validating an HTML document's content involves checking
its markup against a DTD and reporting markup errors.
There are various HTML document types in use on the Web,
arising from various levels and versions.
Each DTD may be recognized by a unique label
called a formal public identifier or FPI
[public use of a DTD implies some measure of universal availability].
When an HTML document is created, the DTD to which it conforms is
declared in a document type declaration at the top of the document.
For example,
<!DOCTYPE HTML PUBLIC
"-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>HTML Validation</TITLE>
</HEAD>
<BODY>
<H1>HTML Validation</H1>
...
</BODY>
</HTML>
(Note that the DOCTYPE line is broken in two only to prevent the example
from being too wide for non-horizontal scrolling - you can keep it all
on one line).
The literal (ie., quoted) text following the word "PUBLIC"
is an FPI referring to the W3C's HTML 4.0 DTD.
This is an easy-to-use HTML validation service based on an SGML parser.
It checks HTML
documents for compliance with W3C HTML Recommendations and other HTML
standards.
To use the W3C HTML 4.0 Recommendation, start your document with one of
the following
doctype declarations:
HTML 4.0 Strict
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
Use this when you want really clean markup,
free of presentational clutter.
Use this together with
Cascading Style Sheets.
HTML 4.0 Transitional
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0
Transitional//EN">
Use this when you need to take advantage of HTML's presentational
features because many of your readers don't have the latest browsers
that understand
Cascading Style Sheets.
HTML 4.0 Frameset
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0
Frameset//EN">
Use this when you want to use HTML
Frames to partition the browser
window into two or more frames.
Content providers can use this service to validate their Web pages
against the HTML 4.0 Recommendation,
thereby ensuring the maximum possible audience for their Web pages.
In addition,
it can be used to check conformance against previous versions of HTML,
including the W3C Recommendation for HTML 3.2
and the IETF HTML 2.0 standard.
To allow authors to broaden their audience even further to those with
disabilities,
the service will be updated according to the guidelines produced
by W3C's Web Accessibility Initiative
(WAI).
You can also test your pages for accessibility using the Web-based
Bobby service.
Half a Loaf is Better Than None
HTML Checkers
HTML Validators and Checkers
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