Web: Lecture 11b

Lecture 11b: XML and Markup Languages

What You Will Learn Today

Compare markup languages.
Create document type definitions.
Create XML DTDs and documents.
Convert HTML documents to XHTML format.

Markup Languages

A markup language defines the structure and appearance of documents and groups of documents.
A metalanguage is a language used to define the rules and symbols of other languages.
XHTML and HTML are designed for specifying the content; appearance is secondary.
The Standard Generalized Markup Language (SGML) is a metalanguage which can define all other markup languages.
SGML is the most general and difficult to follow.

Document Type Definitions (DTDs)

A document type definition (DTD) indicates the markup language used in the document.
- <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" !>
The Strict DTD prevents deprecated elements (tags and attributes).
The Transitional DTD is most commonly used; it allows deprecated HTML elements.
The Frameset DTD is the same as traditional but the document uses <frameset> instead of <body>.

XML

The Extensible Markup Language (XML) is a metalanguage based on SGML but much simpler.
XML rules are more strict and less flexible than HTML.
XML defines the meaning of the content.
XML is used to define languages used to exchange data between different applications.
<!ELEMENT html (head, body)> defines the <html> tag in the HTML DTD.
An XML parser is needed to read and interpret XML elements.
An XML application is needed to create, store, use and share XML documents with other applications.
XML is used for document exchange and connecting systems, especially among big corporations.
XML makes documents more searchable and processable, reducing server loads.
OpenOffice uses XML documents as its native file format (e.g. Writer .sxw, Calc .sxc).
XSL is a stylesheet metalanguage similar to CSS and XML.

XML Example

Create your own book DTD.

<!ELEMENT book (author, title, publisher, year)> <!ELEMENT author (lastname, firstname, middlename?)> <!ELEMENT lastname (#PCDATA)> <!ELEMENT firstname (#PCDATA)> <!ELEMENT middlename (#PCDATA)> <!ELEMENT title (#PCDATA)> <!ELEMENT publisher (#PCDATA)> <!ELEMENT year length CDATA "4">

Use your book DTD to mark up book documents.

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

<book> <author><lastname>Hofstetter</lastname><firstname>Fred</firstname></author> <title>Internet Literacy</title> <publisher>McGraw Hill</publisher> <year>1998</year> </book>

XHTML

XHTML is based on HTML but is compatible with XML format.
XHTML was designed as a way to transition from HTML 4.01 to XML.
XHTML is almost identical to HTML 4.01 but more strict; existing documents must be slightly changed.
The HTML Tidy utility converts HTML documents to XHTML.
You can convert from HTML to XHTML by correcting problems such as the following:

Converting from HTML to XHTML

An XHTML document should begin with a DTD.
Documents must be well-formed (nested). There should be no overlapping tags.
- correct: <p>a paragraph with a word <em>emphasized </em>.</p>
- incorrect: <p>a paragraph with a word <em>emphasized.</p></em>
Some elements cannot be nested in XHTML though XML cannot exclude such nesting.
- <a> tag cannot contain another <a> tag; a <form> cannot contain another <form>, etc.
XML is case-sensitive. Elements and attributes and predefined values must be in lower case.
- correct: <li>
- incorrect: <LI> (which is a different tag)
- correct: <input type="text">
- incorrect: <input type="TEXT">
End tags are required for non-empty elements.
- correct: <p>paragraph 1</p><p>paragraph2</p>
- incorrect: <p>paragraph 1<p>paragraph2
Empty elements must either have an end tag or the start tag must end with />.
- correct: <br/> or <br /> or <br></br>
- incorrect: <br> alone
- Tags without an end tag include br, frame, hr, img, input, link, meta, param.
Attributes must have values.
- correct: <frame noresize="noresize">
- incorrect: <frame noresize>
- Attributes include checked, selected, disabled, multiple, nowrap, noresize,noshade.
Attribute values must always be quoted.
- correct: <td rowspan="3">
- incorrect: <td rowspan=3>
Attributes should use encoded (unescaped) special characters like <, & and >.
- correct: <img src="sl.gif" alt="<Jack&Jill>">
- incorrect: <img src="sl.gif" alt="<Jack&Jill>">
In attributes, leading and trailing white space is removed and multiple spaces are combined into one.
Contents of embedded scripts and styles should be wrapped in a CDATA section.
- <script type="text/javascript"> <![CDATA[... unescaped script content ...
  ]]> </script>

To Do After Class

See Lecture 11a.
Book References: Horstmann Ch. 24, Reding & Vodnik Unit N, Musciano & Kennedy ch. 1, 15, 16
Electronic References: HTML 4 Ch. 3, W3C XHTML and XML