X12c: XHTML Prolog and Head Entities

Instructional Module X12c

XHTML Prolog and Head Entities

Contents

About this Document

Audience and Objectives

Review

Overview
XML Tags

Purpose of the Prolog
XML Declaration
Doctype

HTML Tags

Overview
The XML language specification tells us that well-formed XML documents are made up of three parts: the prolog, an element, possibly ended by miscellaneous comments, processing instructions, or space. The XHTML specification, in turn, says that the element (the second part) must be the `html` element, which in turn is made up of the `head` and the `body` element. The vast majority of what we do is centered on the body element, but this module is an exception: we'll look at the prolog and the `head`. The prolog is common to all XML documents, and so consists of XML tags. The `head` is part of XHTML, and is made up of HTML tags.

Overview

Figure 1: XHTML Document Structure The XML language specification tells us that well-formed XML documents are made up of three parts: the prolog, an element, possibly ended by miscellaneous comments, processing instructions, or space.

The XHTML specification, in turn, says that the element (the second part) must be the html element, which in turn is made up of the head and the body element. The vast majority of what we do is centered on the body element, but this module is an exception: we'll look at the prolog and the head.

The prolog is common to all XML documents, and so consists of XML tags. The head is part of XHTML, and is made up of HTML tags.

The Prolog: XML Tags
Purpose of the Prolog	The Prolog has three purposes: To declare which version of XML is used in this document; To define which character encoding is used; To point to the Document Type Definition for the document.
XML Declaration	The XML Declaration is the first thing that should go in the Prolog, and hence the first thing in the entire document. Here is what the XML Core Working Group says about XML Declarations: [Definition: XML documents should begin with an XML declaration which specifies the version of XML being used.] Let's look at a typical XML declaration: `<?xml version="1.0" encoding="iso-8859-1"?>` <? No question about it ;-) this is the delimiter marking the beginning of an XML tag xml version="1.0" is the string that identifies this file's code and its version. Version 1.0 was released in October 2000, and W3C's XML Working Group announces new versions. encoding="iso-8859-1" tells what character-set this document uses. While this example uses iso-8859-1, the default is UTF-8. (See module W22f for more information about character sets.) ?> The closing delimiter that marks the end of an XML tag.
Doctype	The XML Core Working Group also requires a Document Type Declaration (DTD), which should be the second thing in the document. The DTD contains a machine-readable description of every element in that type of document (in this case, XHTML) and the syntax or grammar for building them into a document. [Definition: The XML document type declaration contains or points to markup declarations that provide a grammar for a class of documents. This grammar is known as a document type definition, or DTD. The document type declaration can point to an external subset (a special kind of external entity) containing markup declarations, or can contain the markup declarations directly in an internal subset, or can do both. The DTD for a document consists of both subsets taken together.] The document type declaration must appear before the first element in the document. Here's a typical XHTML document type declaration: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> Let's look at its pieces: <! This is the opening delimiter for XML documnent-type definition statements DOCTYPE Identifies the line as a document type declaration html This names the root element of the document - recall that an XML document must have exactly one root element PUBLIC Available for general use; SYSTEM document type definitions are also allowed "-//W3C//DTD XHTML 1.0 Transitional//EN" This is the complete definition of the document type, including: -//W3C Name of the developer or authority in charge of this type of document //DTD XHTML 1.0 Transitional Software-readable name of the specific document type, version, and subtype. Transitional is one of three XHTML 1.0 document types; the other two are Strict and Frameset //EN The human-language name "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" The URI of the formal, software-readable Document Type Definition. > Closing tag delimiter

The Prolog: XML Tags

Purpose of the Prolog

The Prolog has three purposes:

To declare which version of XML is used in this document;
To define which character encoding is used;
To point to the Document Type Definition for the document.

XML Declaration

The XML Declaration is the first thing that should go in the Prolog, and hence the first thing in the entire document. Here is what the XML Core Working Group says about XML Declarations:

[Definition: XML documents should begin with an XML declaration which specifies the version of XML being used.]

Let's look at a typical XML declaration:

<?xml version="1.0" encoding="iso-8859-1"?>

<?: No question about it ;-) this is the delimiter marking the beginning of an XML tag
xml version="1.0": is the string that identifies this file's code and its version. Version 1.0 was released in October 2000, and W3C's XML Working Group announces new versions.
encoding="iso-8859-1": tells what character-set this document uses. While this example uses iso-8859-1, the default is UTF-8. (See module W22f for more information about character sets.)
?>: The closing delimiter that marks the end of an XML tag.

Doctype

The XML Core Working Group also requires a Document Type Declaration (DTD), which should be the second thing in the document. The DTD contains a machine-readable description of every element in that type of document (in this case, XHTML) and the syntax or grammar for building them into a document.

[Definition: The XML document type declaration contains or points to markup declarations that provide a grammar for a class of documents. This grammar is known as a document type definition, or DTD. The document type declaration can point to an external subset (a special kind of external entity) containing markup declarations, or can contain the markup declarations directly in an internal subset, or can do both. The DTD for a document consists of both subsets taken together.]
The document type declaration must appear before the first element in the document.

Here's a typical XHTML document type declaration:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

Let's look at its pieces:

<!: This is the opening delimiter for XML documnent-type definition statements
DOCTYPE: Identifies the line as a document type declaration
html: This names the root element of the document - recall that an XML document must have exactly one root element
PUBLIC: Available for general use; SYSTEM document type definitions are also allowed
"-//W3C//DTD XHTML 1.0 Transitional//EN": This is the complete definition of the document type, including:
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd": The URI of the formal, software-readable Document Type Definition.
>: Closing tag delimiter

HTML Tags

HTML Entity

The HTML entity is the root of an XHTML document. This is the object from which all the other entities are derived - the "parent" of all the "child" entities.

Here's an XHTML root entity tag:

<html xmlns="http://www.w3.org/1999/xhtml">

made up of these pieces:

<: opening delimiter for all HTML tags
html: element name - this corresponds to the root element name in the Doctype declaration
xmlns="http://www.w3.org/1999/xhtml": The XML namespace reference. In XML, a namespace is the vocabulary, or list of words, the makes up the XML-derived language. Since anyone can create an XML-based language (it's extensible!) there needs to be a way for browsers to look up the vocabulary and the rules that go with it. That's the purpose of this statement, and the location is give as a standard Web URI.
>: The closing delimiter.

Head Entity

The head and body entities are the two main parts of every (X)HTML file, of course. The body contains everything the browser is supposed to render for people to view - like the stage on which a play is performed. The purpose of the head is to provide a place for preparation - a backstage area, where actors and support crew can get ready for the show. Since browsers are ready to put on the display at a millisecond's notice, contents of the head are all optional. They include:

Title: what shows in the title bar of the display window. Though not necessary, it really looks dumb not to have one!
Stylesheets: CSS and other styles, when they are described internally.
Links: Usually used to link to external stylesheets.
Script functions: program code for script language like JavaScript and ASP. This is only necessary if a page has dynamic features, like rollover buttons, hotspots with popups, or error-checking for forms.
meta entities (generally known as "meta tags"): used for several purposes described below.

Meta Tags

Why meta tags?
According to Webster, meta means "more comprehensive : transcending". Meta tags are for information that is more comprehensive than other tags, transcending the limits of the page itself.

Like everything else in the head entity, a meta tag is not displayed in the browser window. Its purpose is to let coders do a wide range of "behind the scenes" tasks, including:

Listing the author's name or organization
<meta name="author" content="Sarah Strong" />
Giving keywords to help search engines locate the file
<meta name="keywords" content="html xhtml prolog head meta tag" />
Providing a brief description for search engines to display
<meta name="description" content="Prolog and Head Entities of an XHTML file" />
Sending information to the browser, such as the character set in use
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

Meta tags are quite flexible, but they use a limited number of attributes:

id	An identifying name or number
http-equiv	Information normally transmitted in the HTTP protocol
name	Used to name the kind of information the tag contains
content	The information we want the tag to contain
scheme	If this tag is used to contain RDF information, this attribute names the vocabulary source

There are two main types of meta tags:

those that convey information related to the HTTP transfer process, and
those with free-form information named in the tag.

We'll take a look at each of those types here...

http-equiv

Meta tags that have information about the type of file or the transfer process, use the attribute http-equiv. That's because the information is the equivalent to what might be transmitted in the HTTP header sent by the server to the browser.

Here are the major types of HTTP commands whose equivalents are useful in meta tags:

HTTP Command

What It's For

Examples

content-type

Sending information to the browser about what the contents of the file are.
This is useful for older browsers that didn't recognize the XML and DOCTYPE tags.

<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">

refresh

Causing the browser to load a page after a given number of seconds.
This is useful if your page has moved to a new location, and you want to automatically send the viewer to the new page.
The content can also have the time (in seconds) without a URL. In this case, the browser requests the same page to be re-sent.
This is useful for pages with info that changes periodically, such as weather observations or stock market quotes.

<meta http-equiv="Refresh" content="10; URL=http://www.wccnet.edu/finaid.html">

<meta http-equiv="Refresh" content="10">

Name Metatags

Details soon...

About This Document
	Click here for review questions related to this module's objectives.
Audience	This module is for people who know how to create valid, well-formed XHTML code (see module X11c) and are ready to learn about document prolog and head entities.
Objectives	On successful completion of this module, you will be able to: Discuss the purpose of the prolog and head portions of an XHTML file. Define doctypes and identify their different uses Define the purpose of meta information in your XHTML document Declare an encoding type for your document
Module X12c: XHTML Prolog and Head Entities	This document is part of a modular instruction series in Computer Instruction. For more information, see the overview or the list of modules in this series, W: World Wide Web. This document has been used in the following classes: INP 150.
History	Original: 3 October 2003, by Laurence J. Krieg Last modification: Monday, 31-Aug-2009 11:48:07 EDT
Copyright	Copyright © 2003, Laurence J. Krieg, Washtenaw Community College Instructors: You may point to this file in your Web-based materials; however, its location may change without notice. Students: You are welcome to make a copy for your personal use. All other uses: Please contact the author, Laurence J. Krieg, for permission: krieg@ieee.org.