An XML Primer, 5 of 9

How XML Differs From HTML

Like HTML, XML is a subset of SGML (Structured Generalized Markup Language), optimized for delivery over the web.

Unlike HTML, which tags elements in web pages for presentation by a browser, e.g. <bold>Oracle</bold>, XML tags elements as data, e.g. <company>Oracle</company>. For example, you can use XML to give context to words and values in web pages, identifying them as data instead of simple textual or numeric elements.

The following example is in HTML code. This is followed by the corresponding XML example. The examples show employee data:

Employee number
Name
Job
Salary

HTML Example 1

<table>
   <tr><td>EMPNO</td><td>ENAME</td><td>JOB</td><td>SAL</td></tr>
   <tr><td>7654</td><td>MARTIN</td><td>SALESMAN</td><td>1250</td></tr>
   <tr><td>7788</td><td>SCOTT</td><td>ANALYST</td><td>3000</td></tr>
 </table>

XML Example 1

In the XML code, note the addition of XML data tags and the nested structure of the elements.

<?xml version="1.0"?>
  <EMPLIST>
    <EMP>
    <EMPNO>7654</EMPNO>
    <ENAME>MARTIN</ENAME>
     <JOB>SALESMAN</JOB>
     <SAL>1250</SAL>
    </EMP>
    <EMP>
    <EMPNO>7788</EMPNO>
    <ENAME>SCOTT</ENAME>
    <JOB>ANALYST</JOB>
    <SAL>3000</SAL>
    </EMP>
  </EMPLIST>

HTML Example 2

Consider the following HTML that uses tags to present data in a row of a table. Is "Java Programming" the name of a book? A university course? A job skill? You cannot be sure by looking at the data and tags on the page. Imagine a computer program trying to figure this out!

<HTML>
  <BODY>
    <TABLE>
     <TR>
     <TD>Java Programming</TD>
     <TD>EECS</TD>
     <TD>Paul Thompson</TD>
     <TD>Ron<BR>Uma<BR>Lindsay</TD>
     </TR>
    </TABLE>
  </BODY>
 </HTML>

The analogous XML example has the same data, but the tags indicate what information the data represents, not how it should be displayed. It's clear that "Java Programming" is the Name of a Course, but it says nothing about how it should be displayed.

XML Example 2

<?xml version="1.0"?>
  <Course>
      <Name>Java Programming</Name>
    <Department>EECS</Department>
    <Teacher>
      <Name>Paul Thompson</Name>
    </Teacher>
    <Student>
      <Name>Ron</Name>
    </Student>
    <Student>
      <Name>Uma</Name>
    </Student>
    <Student>
      <Name>Lindsay</Name>
    </Student>
  </Course>

XML and HTML both represent information:

XML represents information content
HTML represents the presentation of that content

Summary of Differences Between XML and HTML

Figure 24-2 summarizes, how XML differs from HTML.

Table 24-2 XML and HTML Differences

XML	HTML
Represents information content	Represents the presentation of the content
Has user-defined tags	Has a fixed set of tags defined by standards.
All start tags must have end tags	Current browsers relax this requirement on tags <P>, <B>, and so on.
Attributes must be single or double quoted	Current browsers relax this requirement on tags
Empty elements are clearly indicated	Current browsers relax this requirement on tags
Element names and attributes are case sensitive	Element names and attributes are not case sensitive.