(13) DTDs are not XML Documents

• DTDs use a special syntax
  • somewhat ironic when everything else is XMLized
  • DTDs cannot be processed with standard XML tools
  • more compact than XML syntax
• Definition of elements and attribute lists
  • elements are defined by the content they allow
  • attribute lists are sets of allowed attributes on elements

<!ELEMENT address (name, phone*, website*, postal?)>
<!ELEMENT name (#PCDATA)>
<!ATTLIST name
 short CDATA #REQUIRED
>
<!ELEMENT phone (#PCDATA)>
<!ATTLIST phone
 type ( voice | fax ) #REQUIRED
>
<!ELEMENT postal (#PCDATA)>
<!ELEMENT website (#PCDATA)>

(14) Syntax Rules

• There is no container containing the whole DTD
  • <!ELEMENT xml EMPTY> thus is a complete DTD
• Definitions (officially called declarations) use <!… > syntax
  • ELEMENT is used to define an element [Defining Elements (1)]
  • ATTLIST is used to define an attribute list [Defining Attribute Lists (1)]
  • ENTITY is used to define an entity [Entities (1)]
• The document element is not marked explicitly
  • but it must be declared in the document type declaration
  • this means the document element is defined by the document, not by the DTD

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE address SYSTEM "address.dtd">
<address>

(16) Element Only Content

• Element content is defined by a grammar for the children
  • sequences are indicated with a comma: ,
  • choices are indicated with a vertical bar: |
  • optional parts are indicated with a question mark: ?
  • repeatable parts are indicated with a plus: +
  • optional and repeatable parts are indicated with a asterisk: *
  • parentheses can be used for grouping and nesting

<!ELEMENT table
     (caption?, (col*|colgroup*), thead?, tfoot?, (tbody+|tr+))>
<!ELEMENT caption  %Inline;>
<!ELEMENT thead    (tr)+>
<!ELEMENT tfoot    (tr)+>
<!ELEMENT tbody    (tr)+>
<!ELEMENT colgroup (col)*>
<!ELEMENT col      EMPTY>
<!ELEMENT tr       (th|td)+>
<!ELEMENT th       %Flow;>
<!ELEMENT td       %Flow;>

(17) Mixed Content

• Mixed Content [XML Basics; Mixed Content (1)] allows text content and elements to be mixed
  • Whitespace [XML Basics; Whitespace (1)] characters are allowed in Element Only Content [Element Only Content (1)] (this must not be declared)
  • for non-whitespace characters, character data must be allowed explicitly
• The allowed child elements may be constrained, but not their order or their number of occurrences
• Mixed Content always is defined as <!ELEMENT x (#PCDATA | a | b | …)* >

<!ELEMENT address (#PCDATA | %inline; | %misc.inline; | p)*>

• Character only content is a special case of mixed content
  • the element may only contain characters (no other elements)
  • the repetition is not necessary because there is no choice

<!ELEMENT style (#PCDATA)>

(18) Empty Content

• Empty elements can be useful
  • they may contain all information in attributes
  • their presence may carry semantics without the need for additional information

<!ELEMENT img EMPTY>
<!ATTLIST img
  %attrs;
  src         %URI;          #REQUIRED
  alt         %Text;         #REQUIRED
  name        NMTOKEN        #IMPLIED
  longdesc    %URI;          #IMPLIED
  height      %Length;       #IMPLIED
  width       %Length;       #IMPLIED
  usemap      %URI;          #IMPLIED
  ismap       (ismap)        #IMPLIED
  align       %ImgAlign;     #IMPLIED
  border      %Length;       #IMPLIED
  hspace      %Pixels;       #IMPLIED
  vspace      %Pixels;       #IMPLIED
  >

(20) Attributes belong to Elements

• Attributes are specified in an element's Attribute List
  • an element definition may have any number of attributes associated with it
  • attributes may occur at most once on an element
• Attributes definitions have a name, a type, and a default declaration
  • the attribute appears according to the default declaration
  • if the attribute is present, its value must conform to the type

<!ELEMENT param EMPTY>
<!ATTLIST param
  id          ID             #IMPLIED
  name        CDATA          #REQUIRED
  value       CDATA          #IMPLIED
  valuetype   (data|ref|object) "data"
  type        %ContentType;  #IMPLIED
  >

(21) Attribute Types

• Attribute values can be constrained (which is not possible for element content)
  • CDATA means any character string (but no markup)
  • enumerated types list allowed values: (data|ref|object) (list of XML names)
  • ID for identifying elements (part of ID/IDREF [ID/IDREF (1)])
  • IDREF for referencing identified elements (part of ID/IDREF [ID/IDREF (1)])
• Application-oriented attribute types are often simulated
  • using Parameter Entities [Parameter Entities (1)], modeling information can be preserved

  accept-charset %Charsets;  #IMPLIED

<!ENTITY % Charsets "CDATA">
<!-- a space separated list of character encodings, as per [RFC2045] -->

• The default declaration specifies the attribute's presence
  • #REQUIRED means the attribute has to be specified (on every element)
  • #IMPLIED marks an optional attribute (the parser may imply a value)
  • "…" specifies a default value (and the attribute is optional)

(24) References in Documents

• Without Validation, there are no IDs
  • ID is an attribute type [Attribute Types (1)] declared in the DTD
  • xml:id is an attempt to support schema-independent IDs
• IDs are used to assign identities to elements
  • the XML processor reports duplicate IDs as errors (part of validation [http://www.w3.org/TR/xml/#id])
• IDREFs are used to reference existing IDs
  • the XML processor reports references to non-existing IDs as errors (part of validation [http://www.w3.org/TR/xml/#idref])
• IDs must be XML Names (in particular, they may not start with a number)

(25) ID/IDREF in a Document

<document>
 <section id="sgml" author="dret">
  <title>Standard Generalized Markup Language (SGML)</title>
  <p>SGML is an ISO standard ...</p>
  <section id="sgml-syntax" author="bob">
   <title>SGML Syntax</title>
   <p>SGML uses markup, which is ...</p>
  </section>
 </section>
 <section id="xml" author="bob">
  <title>Extensible Markup Language (XML)</title>
  <p>XML is based on SGML (Section <ref name="sgml"/>) ...</p>
  <p type="example">XML can be used ...</p>
  <section id="xml-syntax" author="dret">
   <title>XML Syntax</title>
   <p>Section <ref name="sgml-syntax"/> describes ...</p>

<!ELEMENT section ( title, p+, section* ) >
<!ATTLIST section
 id ID #REQUIRED
 author CDATA #REQUIRED >
<!ELEMENT title ( #PCDATA )>
<!ELEMENT p ( #PCDATA | ref )*>
<!ATTLIST p
 type CDATA #IMPLIED >
<!ELEMENT ref EMPTY >
<!ATTLIST ref
 name IDREF #REQUIRED >

(26) References within the Tree

(27) Formatting Example

Hotspot can generate links to sections such as the section about ID/IDREF [ID/IDREF (1)], this link is then translated into the appropriate HTML code, meaning a link with the target being a fragment identifier to the slide number.

<p>Hotspot can generate links to sections such as the section about <link href="ididref"/>, this link is then translated into the appropriate HTML code, meaning a link with the target being a fragment identifier to the slide number.</p>

After running Hotspot, the following HTML is generated:

<p>Hotspot can generate links to sections such as the section about <a href="#(23)">ID/IDREF</a>, this link is then translated into the appropriate HTML code, meaning a link with the target being a fragment identifier to the slide number.</p>

(28) ID/IDREF Semantics

• Rooted in the document world
  • all parts are assembled before processing
  • names are symbolic and assigned as required
  • mixed syntax and semantics
• Good idea, but many shortcomings
  • constraints apply to one document only
  • IDs and IDREFs are global instead of scoped
  • identifiers should be allowed to use any type
  • identifier processing should be type-specific (2 ≟ 02)
• Applications must know how to process ID/IDREF
  • for HTML export, links can be generated
  • for databases, keys should be used

(30) General Entities

• XML's core concept of physical data structures
  • an entity is a named unit of data which can be referenced
  • within documents, it is referenced by the markup &entity-name;
• Entities can be used to name and reuse document content

<!ENTITY aacute "&#225;"> <!-- latin small letter a with acute,
                                  U+00E1 ISOlat1 -->
<!ENTITY acirc  "&#226;"> <!-- latin small letter a with circumflex,
                                  U+00E2 ISOlat1 -->
<!ENTITY atilde "&#227;"> <!-- latin small letter a with tilde,
                                  U+00E3 ISOlat1 -->
<!ENTITY auml   "&#228;"> <!-- latin small letter a with diaeresis,
                                  U+00E4 ISOlat1 -->

• Character References look like entities: ☺ or ☺ = ☺
  • they can be used to represent any Unicode character, they are processed as single characters

(31) Parameter Entities

• Parameter entities are parsed entities for use within the DTD
  • a parameter entity must be specifically declared as such
  • within DTDs, it is referenced by the markup %entity-name;
  • outside of DTDs, parameter entities cannot be used
• As general entities, parameter entities are meant for reuse
  • in a DTD, reuse is mostly about reusing structures
  • parameter entities are DTDs duct tape, not elegant, but effective

(32) XHTML Parameter Entities (Attributes)

<!ELEMENT p %Inline;>
<!ATTLIST p
  %attrs;
  %TextAlign;
  >

<!ENTITY % attrs "%coreattrs; %i18n; %events;">

<!ENTITY % coreattrs
 "id          ID             #IMPLIED
  class       CDATA          #IMPLIED
  style       %StyleSheet;   #IMPLIED
  title       %Text;         #IMPLIED"
  >

<!ENTITY % i18n
 "lang        %LanguageCode; #IMPLIED
  xml:lang    %LanguageCode; #IMPLIED
  dir         (ltr|rtl)      #IMPLIED"
  >

<!ENTITY % LanguageCode "NMTOKEN">
<!-- a language code, as per [RFC3066] -->

<!ENTITY % TextAlign "align (left|center|right|justify) #IMPLIED">

(33) XHTML Parameter Entities (Content)

<!ELEMENT p %Inline;>
<!ATTLIST p
  %attrs;
  %TextAlign;
  >

<!ENTITY % Inline "(#PCDATA | %inline; | %misc.inline;)*">

<!ENTITY % inline "a | %special; | %fontstyle; | %phrase; | %inline.forms;">

<!ENTITY % special
   "%special.basic; | %special.extra;">

<!ENTITY % special.basic
 "br | span | bdo">

<!ENTITY % special.extra
   "object | applet | img | map | iframe">

<!ENTITY % misc.inline "ins | del | script">