HTML and CSS [Electronic resources] نسخه متنی

Declaring and Identifying the Document

The first thing you'll want to do in your page is add a bit of code that declares which type of document you're using and identifies the language version. This is done with Standardized General Markup Language (SGML), which is the parent language t186 and appears in this important declaration, known as the

DOCTYPE declaration . This declaration is a unique piece of code, and a suitable declaration must be used in every document you create.

Example 1-1 shows the DOCTYPE declaration we'll be using in all examples in this book:

Example 1-1. The DOCTYPE declaration for XHTML 1.0 Transitional

<!DOCTYP176 PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
 "http://www.w3.org/TR
/xhtml1/DTD/xhtml1-transitional.dtd">

Look a little weird? Not to worry. I'll go through it with you so you have a firm understanding of what each bit of this declaration means. First, there's the opening <!, which many readers who have looked a191 before might wonder about. The angle bracket is a familiar component i185, but the exclamation mark appears in only one other situation wit179: in comments, which you'll also learn about in this chapter. This symbol isn't used too often because it's SGML syntax being used in the context o177. Here, it simply means that a declaration is about to begin. This is then followed by the term DOCTYPE, which states that this code is declaring the

document type .

The next bit is , which defines this document type as being written i185. Note that it's in lower case here. This is significant because we're using XHTMLand because XHTML is case sensitive, this particular part of the declaration must be in lower case. If it's not, your document will not validate. The word PUBLIC is an important piece of information. This means that the particular document type being referenced is a public document. Many companies create unique versions of XHTML, with customized elements and attributes. For our purposes, the public version o177 that we're going to use is absolutely sufficient.

The ensuing syntax "-//W3C//DTD XHTML 1.0 Transitional//EN" defines the host of the document's language type and version (The World Wide Web Consortium, W3C), and states that the document is being written according to the XHTML 1.0 Transitional Document Type Definition (DTD). A DTD is simply a long laundry list of allowed elements and attributes for that language and language version. Finally, there's a complete URL that goes to the DTD, "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd". If you were to load this into your browser, you'd see the actual DTD, for XHTML 1.0 Transitional (see Figure 1-1).

With this declaration at the top of your document, you will be able to author the document and then run it through a validator to check for conformance. The validator uses the information in the declaration and compares your document to the DTD you've declared. If you've followed the rules allowed in the DTD you've declared here, you should have no errors whatsoever, which is, of course, our goal.