DTD - Element Comparison with Attribute

The main building blocks of XML and HTML documents are tags like <body>....</body>.

XML document building blocks

All XML documents (and HTML documents) are composed of the following simple building blocks:

  • Element
  • Attribute
  • Entity
  • PCDATA
  • CDATA

The following is a brief description of each building block.

Element

Elements are the main building blocks of XML and HTML documents.Main building blocks.

Examples of HTML elements are "body" and "table". Examples of XML elements are "note" and "message". Elements can contain text, other elements, or be empty. Examples of empty HTML elements are "hr", "br", and "img".

Example:

<body>body text in between</body>
<message>some message in between</message>

Attribute

Attributes can provideAdditional information about the element.

Attributes are always placed within the start tag of an element. Attributes are always preceded byName/ValueThe form is paired. The following "img" element has additional information about the source file:

<img src="computer.gif" />

The name of the element is "img". The name of the attribute is "src". The value of the attribute is "computer.gif". Since the element itself is empty, it is closed with a "/".

Entity

Entities are used to define variables for common text. Entity references are references to entities.

Most students are familiar with this HTML entity reference: " ". This 'non-breaking space' entity is used in HTML to insert an additional space in a document.

Entities will be expanded when the document is parsed by the XML parser.

The following entities are predefined in XML:

Entity reference Character
< <
> >
& &
" "
' '

PCDATA

PCDATA stands for parsed character data (parsed character data).

Character data can be imagined as the text between the start and end tags of an XML element.

PCDATA is the text that will be parsed by the parser. These texts will be checked for entities and markers by the parser.

Tags in the text will be treated as markers, while entities will be expanded.

However, the parsed character data should not contain any &、< or > characters; they should be replaced with &、< and > entities respectively.

CDATA

CDATA stands for Character Data.

CDATA is text that will not be parsed by the parser.The tags in these texts will not be treated as markup, and the entities will not be expanded.