For now, it’s important that you code your HTML correctly, so that your Web documents will display correctly in the future when XML is used more frequently.
The global transition from HTML to XML is having a significant impact on Web document coding and markup. The standards for developing HTML-based documents are getting “stricter” – that is, HTML-based documents and applications are being written in more specific ways.
To prepare for this transition, there is a new markup standard known as XHTML. XHTML is almost identical to HTML 4.01, except the syntax and organization of code is more precise. For example:
1) XHTML requires that tag names must be lowercase (e.g. <body> instead of <BODY>)
2) XHTML requires that code must be "well-formed" - that is, tags must be properly nested. You must provide the appropriate number of open and closed parentheses in the correct order.
3) XHTML requires that all tags must be closed - that is, each non-empty tag must have a closing tag.
Many Web page editors - including Dreamweaver - automatically apply "strict" HTML formatting. However, you should always check your code to make sure it is correctly written. One way to make sure that your code is correctly written is to use the W3 Markup Validation Service available at http://validator.w3.org/.
This utility will automatically review your HTML and notify you if your code is not correctly written. Your code doesn't necessarily have to be written in XHTML to be correct - correct HTML documents will do just as well.
If you'd like more information
about XHTML requirements, consult the section of this tutorial entitled "additional resources".