XML and the Web

R. Alexander Miłowski

milowski@ischool.berkeley.edu

School of Information, UC Berkeley

What is XML?

XML is a syntax. HTML is a vocabulary.

XML is:

a syntax for markup,
where you get to make up your own names,
for elements and attributes,
with a baseline syntax for what is a well-formed document.

Some Well-Known XML Vocabularies

SVG
MathML
XHTML
DocBook - books
TEI - Text Encoding Initiative

A Complete Example

The source document.

<?xml version="1.0" encoding="UTF-8"?>
<article xmlns="http://docbook.org/ns/docbook"
    xmlns:xlink="http://www.w3.org/1999/xlink"
    version="5.0">
    <title>A Sample Document</title>
    <section>
        <title>Some Mathematics</title>
        <para>Here is an unknown equation: <equation>
            <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="block">
               <mml:semantics>
                  <mml:mrow>
                     <mml:mrow>
                        <mml:mi mathvariant="bold">a</mml:mi>
                        <mml:mo>=</mml:mo>
                        <mml:mfrac>
                           <mml:mrow>
                              <mml:mi mathvariant="bold">F</mml:mi>
                           </mml:mrow>
                           <mml:mi>m</mml:mi>
                        </mml:mfrac>
                        <mml:mo>=</mml:mo>
                        <mml:mfrac>
                           <mml:mrow>
                              <mml:mi>q</mml:mi>
                              <mml:mo>[</mml:mo>
                              <mml:mi mathvariant="bold">E</mml:mi>
                              <mml:mo>+</mml:mo>
                              <mml:mfenced>
                                 <mml:mrow>
                                    <mml:mi mathvariant="bold">v</mml:mi>
                                    <mml:mi>X</mml:mi>
                                    <mml:mi mathvariant="bold">B</mml:mi>
                                 </mml:mrow>
                              </mml:mfenced>
                              <mml:mo>]</mml:mo>
                           </mml:mrow>
                           <mml:mi>m</mml:mi>
                        </mml:mfrac>
                     </mml:mrow>
                  </mml:mrow>
               </mml:semantics>
            </mml:math>
         </equation>
        </para>
    </sect1>
</article>

An More Complicated Example

An extract from the US State Department

   <text>
        <front>
            <titlePage type="main">
                <pb n="I" xml:id="pg_I" facs="0001"/>
                <figure>
                    <graphic url="figure_0001.tif"/>
                </figure>
                <docTitle>
                    <titlePart type="series">Foreign Relations of the United States</titlePart>
                    <titlePart type="subseries">1945–1950</titlePart>
                    <titlePart type="volume">Emergence of the Intelligence Establishment</titlePart>
                </docTitle>
                <docImprint>Department of State<lb/>Washington, DC</docImprint>
                <byline>Editor: <persName>C. Thomas Thorne, Jr.</persName>
                    <lb/>
                    <persName>David S. Patterson</persName>
                </byline>
                <byline>General Editor: <persName>Glenn W. LaFantasie</persName>
                </byline>
                <docImprint>
                    <publisher>United States Government Printing Office</publisher>
                    <pubPlace>Washington</pubPlace>
                    <pb n="II" xml:id="pg_II" facs="0002"/>
                    <docDate>1996</docDate>DEPARTMENT OF STATE PUBLICATION 10316<lb/>OFFICE OF THE
                    HISTORIAN<lb/>BUREAU OF PUBLIC AFFAIRS<lb/>For sale by the U.S. Government
                    Printing Office Superintendent of Documents, Mail Stop: SSOP, Washington, DC
                    20402-9328<lb/>ISBN 0-16-045208-2</docImprint>
            </titlePage>
            <pb n="III" xml:id="pg_III" facs="0003"/>
            <div type="section" xml:id="preface">
                <head>Preface</head>
                <p>The <hi rend="italic">Foreign Relations</hi> of the United States series presents
                    the official documentary historical record of major foreign policy decisions and
                    significant diplomatic activity of the United States Government. The series
                    documents the facts and events that contributed to the formulation of policies
                    and includes evidence of supporting and alternative views to the policy
                    positions ultimately adopted.</p>

Well-formed XML Documents

all elements must be closed,
and balanced (e.g. start and end tag names match)
attribute are not duplicated and have properly quoted values,
no un-escaped syntax characters (e.g. '<' or '&')
entities are defined (e.g. <, &)

Naming in XML

Purpose: to differentiate your use of names from mine.

Is address a street location or a computer IP address?

names in XML can be qualified by URIs,
a prefixed can be declared as a mapping for that URI,
or you can default the namespace URI.
The combination of a prefix and local name resolves to a pair of a URI and a name.
This allows names to be scoped.

Syntax of Namespaces

Names are declared via attributes:

<doc xmlns="http://example.com/doc" xmlns:m="http://www.w3.org/1998/Math/MathML"/>

Declaration are scoped to the descendants:

<doc xmlns="http://example.com/doc" xmlns:m="http://www.w3.org/1998/Math/MathML">
   <p>I'm {http://example.com/doc}p</p>
   <m:math>I'm {http://example.com/doc}math</m:math>
   <b:error>I'm broken!</b:error>
</doc>

Name resolve via the ancestors:

<doc xmlns="http://example.com/doc" xmlns:m="http://www.w3.org/1998/Math/MathML">
   <div xmlns:f="http://example.com/functions">
      <f:function name="binomial">
         <m:math>
          <m:apply><m:csymbol cd="relation1">eq</m:csymbol>
           <m:apply><m:csymbol cd="combinat1">binomial</m:csymbol><m:cn>4</m:cn><m:cn>2</m:cn></m:apply>
           <m:cn>6</m:cn>
          </m:apply>
         </m:math>         
      </f:function>
   </div>               
</doc>

Why didn't XML take over the world?

Answer #1: It did. You just don't see it.

Answer #2: It isn't an intrinsic vocabulary. It's a syntax.

Answer #3: Web browsers don't do the same thing with XML documents as they do with HTML.

Impoverished Processing Model

When the browser loads an XML document:

scripts do not run,
links are not identified,
embedded objects to not load.

The result is applications do not work well.

This can be fixed:

there are standards,
but it is a little late,
and browser vendors aren't interested.