Introduction

One of the most important discussions in this section is the last paragraph on p. 81 about separating presentation from structure. Be sure to read over this pargraph, because I may ask a test question based on this concept. If there is anything in the paragraph you don't understand, ask about it in lecture or via email.

XHTML is a markup language. A key issue for XHTML is separating presentation from structure. HTML often mixed (and still does mix) content, structure, and formatting. XHTML 1.1 allows only the specification of content and structure, not formatting. For XHTML documents, formatting should be specified via Cascading Style Sheets (CSS).

Editing XHTML We edit documents in source-code form. We use a text editor to edit HTML documents (e.g. notepad or wordpad on Windows) vi and emacs are text editors commonly used on Unix systems. XHTML documents have filenames ending in .htm or .html Web servers are machines that store XHTML documents and make them available over the Internet.

First XHTML Example

Be sure to look over the first XHTML example on p. 82. Most things about this example are the same as the examples we've been seeing in class. A few things are different.

First, the following appears at the top of the file:

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The text points out that the meaning of these lines is explained in Chapter 20. Specificaly, the first line is explained on p.635, and the second line is explained (in part) on p. 637.

Here's a short explanation of this part:

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This says that the document conforms to the XML version 1.0 standard, which is a "superset" of XHTML.

When we say that XHTML is a superset of XML, we mean that:

everything that is XHTML is also XML
not everything that is XML is necessarily XHTML

XML is a more general standard than XHTML, and is used for structuring all kinds of data. In fact, these reading notes start out in XML, and get converted to XHTML by your web browser. XML allows users to define their own tags for customized applications, or use predefined tags like those in XHTML. Chapter 20 has lots more information on XML if you are interested.

For conforming with standards, we also see an attribute on the html element called xmlns, which stands for "XML namespace". Again, the detailed discussion is in Chapter 20; the short version is that namespaces allow lots of folks to come up with names for tags without worrying that two folks will use the same tag name for different purposes. If this happens (by accident, or on purpose), the namespace attribute and some special codes can be used to make sure that the meaning of the XHTML code (or XML code) is clear.

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Use and ]]> to mark comments in XHTML. Know the html, head, title and body tags. Know that the head element may contain style sheets and scripts, as well as the title element. Know the terms tag, element, open tag, close tag, attributes and nested elements. Tags in XHTML must be properly nested (see Common Programming Error 4.3).

W3C XHTML Validation Service A validation service can be used to ensure that a website is syntactically correct, i.e. that it follows all the rules of XHTML. Most browsers are very forgiving of syntax errors in HTML or XHTML code. However, XHTML and browsers themselves will probably only grow more complex over time. The best way to sure your code will work properly in many different browsers is to be sure that it follows the standards exactly. validator.w3.org is one web site that can be used to validate XHTML.

Headers

Linking

Images

Special Characters and More Line Breaks

Unordered Lists

Nested and Ordered Lists

Web Resources