Lecture 3

XML and XHTML

Ketan Mayer-Patel

University of North Carolina

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Assignment 1

Character References (Entities)

The problem
Most commonly used references:
- & for &
- < for <
- > for >
- " for "
General numeric character reference form
- &#nnn;
Example

Back to document type.

DTD indicates standard to follow.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
- Identifying string
- URL pointing to a copy of the official document type definition
Flavors of HTML
- Strict
- Transitional
- Frameset
The official HTML 4.0 standard

Validation

Type declaration is machine readable
Specifies legal elements, attributes, content constraints, etc.
Advantages of validation
Tool for validating HTML
- W3C Validation Service
A word about character sets.
- Revised minimum HTML example.

HTML shortcomings

Mixed case
Inconsistent nesting
- Empty tags and implied tags.
- Ambiguous validation errors.
Attribute quoting
Closed tag set for a specific domain

Motivating XML

Separate structure from semantics
- Allow domain-specific tags
- Provide a way to mix tag vocabularies
- Use it for more than the just the web.
Enforce good structure
- Case sensitivity
- Perfect nesting
- Quoted attributes
XHTML 1.0 = HTML 4.0 + XML
- Abide by the rules of XML for structure
- Meaning of tags provided by HTML 4.0

XML Overview

Format for structured documents
- Structure model = heirarchically nested elements
Domain-specific element tags
- Meaning determined by common agreements and standards
- Separates form from meaning

XML Standards

Thin, purpose-specific standards

XML Document Structure

XML Declaration
- Required
Document Type Declaration
- Generally optional, but needed for XHTML
Document Body
- Required

XML Declaration

<?xml version="1.0" encoding="UTF-8" ?>

Must be first line of the document
- Nothing can come before (not even whitespace)
version attribute is required
encoding is optional but widely used

Examples

Valid declarations:

<?xml version="1.0" ?>

<?xml version="1.0" encoding ="1.0" ?>

Invalid declarations:

<xml version="1.0" ?>

<?xml version="1.0" >

<?XML version="1.0" ?>

<?xml version=1.0 ?>

DTD Declaration

<!DOCTYPE Root_Element SYSTEM Public_ID DTD_URL?>

Specifies encompassing root element
ID string is unique and well-known
DTD URL not guaranteed to work
DTD form is compatible with HTML 4.0 DTDs

XML Document Body

Mix of markup elements and content.
- Content also known as character data.
Root element encloses all.

XML Elements

Start tag form:

<element_name attributes*>

End tag form:

</element_name>

Element name rules
- Case sensitive
- Starts with letter, underscore or colon
- May contain letters, underscore, colon, digits, hyphen, or period.

XML Attributes

Form

name="value"

name='value'

Attribute name rules same as for elements
Values MUST be quoted
Value MUST be given
Difference with HTML

Reserved Attributes

Commonly used:
- xml:lang
  - Language/character set information for an element
- xmlns
  - Namespace mechanism
- xml:base
- xml:space

An Example XML document

A minimal XML document

Empty Elements

Special element tag syntax for empty elements

<element_name attributes* />

Example:

<br />

<br id="break1" />

XML Comment Mechanism

Start of comment: <!--
End of comment: -->
Comment content ignored.
- No meaning as either markup or character data
Restrictions:
- Cannot contain --
- No nesting.
- Cannot be inside start, end, or empty tags

XML Entities

Same mechanism as HTML character references
Numeric character value entity forms:

&#decimal_value;

&#xhex_value;

General XML defines only 5 by name:
- <, >, &, &quot, '
- <, > & ", &apos
Specific uses of XML can define additional named entities

CDATA

Useful when character data (i.e., non-markup content) contains lots of special characters.
Start of CDATA: <![CDATA[
End of CDATA: ]]>
Whatever is inside is not processed for markup
- Means that we don't have to use entities for &, <, >, etc.
Example

Well-Formedness

XML provides rules for structure (not meaning)
Conforming documents known as "well-formed"
Rules for well-formedness:
- Must start with xml declaration.
- Tags must next perfectly
 - Good: <a></a>
 - Bad: <a></a>
- Empty tags must use empty tag form:
 - Example: 
- All attributes must have values
- All attribute values must be quoted
- All elements must be enclosed by a single root tag.

Sharing Vocabularies

Suppose we wanted to invent a set of XML tags for a bookstore.
- What elements might we define?
- How might attributes be?
What problems come up if I want to publish or distribute my XML files which describe my books?

Sharing XML Problems

P1: Meaning of tags are specific to whomever invented them.
P2: Different people/organizations my use the same tag names.
- Our bookstore vs Library of Congress vs Amazon vs...
Solutions?

Sharing XML Solutions

Solution to P1: Standardization and documentation.
- Create standardized description for what particular tag vocabularies mean.
  - XML Standards Picture
- Still have problem of different tag sets using the same names
Solution to P2: Namespaces

Namespace

Motivation:

Define a mechanism for unqiuely naming elements and attributes so different vocabularies can be mixed into an XML document without name conflicts.
-- Sall, XML Family of Specifications, p. 211.

XML Namespaces Overview

Tag vocabularies identified with a unique URL
- URL does not have to point to anything.
  - Often points to explaining documentation or website of organization
- Simply used as a unique string to identify the tag vocabulary
Associate a prefix with the namespace.
Use qualified element and attribute names

Qualified Names

Form

prefix:name

Prefix given in namespace declaration.
Name is element or attribute name defined within namespace.
Examples:
- xhtml:ul
- xlink:href
- kmp:book
- foo:bar

Declaring a Namespace

xmlns attribute
- Form: xmlns:prefix="ID_URL"
- Example: <kmp:bookstore xmlns:kmp="http://kmp-books.com/xml-tags">
Associates tags and attributes with prefix
- Prefix used in qualified element and attribute names
- Prefix can be almost anything
  - Should not be "xml"

Namespace Prefix Scope

Scope of prefix is element where declared
Applies immediately
Example

Default namespace

Useful if most/all tags are part of the same tag vocabulary
Form: xmlns="ID_URL"
Example:

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

Validating XML

XML demands well-formedness
Meaning of elements/attributes specific to application domain
Validity defined by some other mechanism
- One mechanism: DTD
- Others: XML-Scheme, RELAXNG, Schematron
- XML schema language comparison

XHTML (at last!)

XHTML 1.0 = XML using tags defined by HTML 4.0
- Namespace identified by this URL:

http://www.w3.org/1999/xhtml

Format specific tags and attributes deprecated
DTD's defined for different levels of conformity for validation.

XHTML DTDs

Strict

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

Transitional and Frameset DTDs also defined
Correspond to HTML 4.0 DTDs (but the DTDs are not the same!)

Important XHTML Rules

Must be well-formed XML
- Perfect nesting
- Quoted attribute values
- Empty element form
Must specify one of the three XML DTDs
Must declare XHTML namespace
Element and attribute names are all lowercase.
- Small but important difference with HTML 4.0

COMP 416, Fall 2010

XML and XHTML

Lecture 3

XML and XHTML

Ketan Mayer-Patel

University of North Carolina

Announcements

Character References (Entities)

Back to document type.

Validation

HTML shortcomings

Motivating XML

XML Overview

XML Standards

XML Document Structure

XML Declaration

Examples

DTD Declaration

XML Document Body

XML Elements

XML Attributes

Reserved Attributes

An Example XML document

Empty Elements

XML Comment Mechanism

XML Entities

CDATA

Well-Formedness

Sharing Vocabularies

Sharing XML Problems

Sharing XML Solutions

Namespace

XML Namespaces Overview

Qualified Names

Declaring a Namespace

Namespace Prefix Scope

Default namespace

Validating XML

XHTML (at last!)

XHTML DTDs

Important XHTML Rules

Examples

Learning and Using XHTML

State of XHTML Today