3.2 Elements, tags, and attributes

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

　　Your 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.

　　You might like to think of this as “chunking” content. At the very top level you have one chunk, the book. Look a little deeper, and you have more chunks, the individual chapters. These are chunked further into paragraphs, footnotes, character names, and so on.

　　Notice how you can make this differentiation between different elements of the content without resorting to any SGML terms. It really is surprisingly straightforward. You could do this 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 SGML (HTML, 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 DTD 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>.

<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>

　　Not all elements require an end tag. Some elements have no content. For example, in HTML you can indicate that you want a horizontal line to appear in the document. Obviously, this line has no content, so just the start tag is required for this element.

<p>This is a paragraph.</p>

<hr>

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

　　If it is not obvious by now, 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.

<p>This is a simple <em>paragraph</em> where some
  of the <em>words</em> have been <em>emphasized</em>.</p>

　　The DTD will specify the rules detailing which elements can contain other elements, and exactly what they can contain.

　　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 sufficiently recent versions of HTML, the <p> element has an attribute called align, which suggests an alignment (justification) for the paragraph to the program displaying the HTML.

　　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.

<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 will only take specific values, such as left or justify. Others will allow you to enter anything you want. If you need to include quotes (") within an attribute then use single quotes around the attribute value.

<p align='right'>I am on the right!</p>

　　Sometimes you do not need to use quotes around attribute values at all. However, the rules for doing this are subtle, and it is far simpler just to always quote your attribute values.

　　The information on attributes, elements, and tags is stored in SGML catalogs. The various Documentation Project tools use these catalog files to validate your work. The tools in textproc/docproj include a variety of SGML catalog files. The FreeBSD Documentation Project includes its own set of catalog files. Your tools need to know about both sorts of catalog files.