7.2. Elements, Tags, and Attributes

All the vocabularies written in XML share certain characteristics. This is hardly surprising, as the philosophy behind XML will inevitably show through. One of the most obvious manifestations of this philosophy is that of content and elements.

Documentation, whether it is a single web page, or a lengthy book, is considered to consist of content. This content is then divided and further subdivided into elements. The purpose of adding markup is to name and identify the boundaries of these elements for further processing.

For example, consider a typical book. At the very top level, the book is itself an element. This book element obviously contains chapters, which can be considered to be elements in their own right. Each chapter will contain more elements, such as paragraphs, quotations, and footnotes. Each paragraph might contain further elements, identifying content that was direct speech, or the name of a character in the story.

It may be helpful to think of this as chunking content. At the very top level is one chunk, the book. Look a little deeper, and there are more chunks, the individual chapters. These are chunked further into paragraphs, footnotes, character names, and so on.

Notice how this differentiation between different elements of the content can be made without resorting to any XML terms. It really is surprisingly straightforward. This could be done with a highlighter pen and a printout of the book, using different colors to indicate different chunks of content.

Of course, we do not have an electronic highlighter pen, so we need some other way of indicating which element each piece of content belongs to. In languages written in XML (XHTML, DocBook, et al) this is done by means of tags.

A tag is used to identify where a particular element starts, and where the element ends. The tag is not part of the element itself. Because each grammar was normally written to mark up specific types of information, each one will recognize different elements, and will therefore have different names for the tags.

For an element called element-name the start tag will normally look like <element-name>. The corresponding closing tag for this element is </element-name>.

Example 7.1. Using an Element (Start and End Tags)

XHTML has an element for indicating that the content enclosed by the element is a paragraph, called p.

<p>This is a paragraph.  It starts with the start tag for
  the 'p' element, and it will end with the end tag for the 'p'
  element.</p>

<p>This is another paragraph.  But this one is much shorter.</p>

Some elements have no content. For example, in XHTML, a horizontal line can be included in the document. For these empty elements, XML introduced a shorthand form that is completely equivalent to the two-tag version:

Example 7.2. Using an Element Without Content

XHTML has an element for indicating a horizontal rule, called hr. This element does not wrap content, so it looks like this:

<p>One paragraph.</p>
<hr></hr>

<p>This is another paragraph.  A horizontal rule separates this
  from the previous paragraph.</p>

The shorthand version consists of a single tag:

<p>One paragraph.</p>
<hr/>

<p>This is another paragraph.  A horizontal rule separates this
  from the previous paragraph.</p>

As shown above, elements can contain other elements. In the book example earlier, the book element contained all the chapter elements, which in turn contained all the paragraph elements, and so on.

Example 7.3. Elements Within Elements; em
<p>This is a simple <em>paragraph</em> where some
  of the <em>words</em> have been <em>emphasized</em>.</p>

The grammar consists of rules that describe which elements can contain other elements, and exactly what they can contain.

Important:

People often confuse the terms tags and elements, and use the terms as if they were interchangeable. They are not.

An element is a conceptual part of your document. An element has a defined start and end. The tags mark where the element starts and ends.

When this document (or anyone else knowledgeable about XML) refers to the <p> tag they mean the literal text consisting of the three characters <, p, and >. But the phrase the p element refers to the whole element.

This distinction is very subtle. But keep it in mind.

Elements can have attributes. An attribute has a name and a value, and is used for adding extra information to the element. This might be information that indicates how the content should be rendered, or might be something that uniquely identifies that occurrence of the element, or it might be something else.

An element's attributes are written inside the start tag for that element, and take the form attribute-name="attribute-value".

In XHTML, the p element has an attribute called align, which suggests an alignment (justification) for the paragraph to the program displaying the XHTML.

The align attribute can take one of four defined values, left, center, right and justify. If the attribute is not specified then the default is left.

Example 7.4. Using an Element with an Attribute
<p align="left">The inclusion of the align attribute
  on this paragraph was superfluous, since the default is left.</p>

<p align="center">This may appear in the center.</p>

Some attributes only take specific values, such as left or justify. Others allow any value.

Example 7.5. Single Quotes Around Attributes
<p align='right'>I am on the right!</p>

Attribute values in XML must be enclosed in either single or double quotes. Double quotes are traditional. Single quotes are useful when the attribute value contains double quotes.

Information about attributes, elements, and tags is stored in catalog files. The Documentation Project uses standard DocBook catalogs and includes additional catalogs for FreeBSD-specific features. Paths to the catalog files are defined in an environment variable so they can be found by the document build tools.

7.2.1. To Do…

Before running the examples in this document, install textproc/docproj from the FreeBSD Ports Collection. This is a meta-port that downloads and installs the standard programs and supporting files needed by the Documentation Project. csh(1) users must use rehash for the shell to recognize new programs after after they have been installed, or log out and then log back in again.

  1. Create example.xml, and enter this text:

    <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
    
    <html xmlns="http://www.w3.org/1999/xhtml">
      <head>
        <title>An Example XHTML File</title>
      </head>
    
      <body>
        <p>This is a paragraph containing some text.</p>
    
        <p>This paragraph contains some more text.</p>
    
        <p align="right">This paragraph might be right-justified.</p>
      </body>
    </html>
  2. Try to validate this file using an XML parser.

    textproc/docproj includes the xmllint validating parser.

    Use xmllint to validate the document:

    % xmllint --valid --noout example.xml

    xmllint returns without displaying any output, showing that the document validated successfully.

  3. See what happens when required elements are omitted. Delete the line with the <title> and <//title> tags, and re-run the validation.

    % xmllint --valid --noout example.xml
    example.xml:5: element head: validity error : Element head content does not follow the DTD, expecting ((script | style | meta | link | object | isindex)* , ((title , (script | style | meta | link | object | isindex)* , (base , (script | style | meta | link | object | isindex)*)?) | (base , (script | style | meta | link | object | isindex)* , title , (script | style | meta | link | object | isindex)*))), got ()

    This shows that the validation error comes from the fifth line of the example.xml file and that the content of the <head> is the part which does not follow the rules of the XHTML grammar.

    Then xmllint shows the line where the error was found and marks the exact character position with a ^ sign.

  4. Replace the title element.

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