2) Ensure Compatibility - Try to make your HTML code compatible with as many internet browsers as possible. Right now, Big Two are Microsoft Internet Explorer (MSIE) and Mozilla Firefox, but it is useful to optimize for older versions of these browsers (many new HTML tags are incompatible with these browsers), as well as other browsers like Opera and Netscape.

3) Eliminate Excessive Whitespace Excessive whitespace is unnessary and keeps your content lower on page than is necessary. Eliminating excessive whitespace is therefore marginally useful to Search Engine Optimization.

Tags (brackets, greater than, less than):

Just like in HTML, you use greater than (">") and less than ("<") signs called tags to indicate opening and closing of an element.

Hank Aaron Babe Ruth

Elements:

Elements are basic building blocks of XML. They may contain text, comments, or other elements, and consist of a start tag and an end tag. Typically, XML elements are akin to nouns in real world. They represent people, places, or things.

Hank Aaron Babe Ruth

Note that in XML, every opening element (i.e. "") must also contain a closing element (i.e. ""). The closing element consists of name of opening element, prefixed with a slash ("/"). XML is case-sensitive. While "" is well-formed, "" and "" are not. Also, if element does not contain text or other elements, you may abbreviate closing tag by simply adding a slash ("/") before closing bracket in your element (i.e. "" can be abbreviated as ""). In addition to rules defining opening and closing tags, it is important to note that in order to create a well-formed XML document, you must properly nest all elements. The previous document properly nests "" elements within "" element, but following would not be acceptable in XML because second "" element exists outside of "" element:

Hank Aaron Babe Ruth

Attributes:

Where elements represent nouns contained in an XML document, attributes represent adjectives that describe elements. The following document tells me that Hank Aaron's id is "1" and that Babe Ruth's is "2". This helps to describe these two employees.

Hank Aaron Babe Ruth

Note that in order to be well formed, all attribute values must be contained within quotation marks. id="1" is correct, while id=1 is not acceptable. This is a marked difference from standard HTML formatting that places much looser restrictions on what is acceptable.

Text/Content:

Elements contain contents that give critical information about them. This information represents that entity itself in an XML document. In following document, Hank Aaron is employee; Babe Ruth is employee. Hank Aaron Babe Ruth

As you can see, XML and HTML are practically identical with exception that XML is far less lenient when it comes to case-sensitivity, using closing tags, and properly nesting parent/child elements. This is excellent news for Web developers everywhere as it ensures that if you write well-formed HTML, you'll find transition to XML virtually seamless.

To summarize, XML is a text-based meta-language that uses tags, elements, and attributes to add structure and definition to documents. It is similar to HTML in syntax and implementation, but different with regard to functionality. Where HTML allows users to control how documents are displayed, XML allows them to describe actual contents of documents. It is a markup language because it uses tags to mark-up documents and it is a meta-language because it uses these tags to give structure to documents that it in turn uses as a means of communication. XML is extensible because it enables users to create their own collection of tags (unlike HTML).

Now, why should you care about XML? If for now other reason, consider that World Wide Web Consortium (W3C), Internet's governing body, is considering a proposal to rewrite HTML 4 language in XML 1.0. As of time this article was written, XHTML had received endorsement by director of W3C as a recommendation. This proposal, known as XHTML will require well formedness in all HTML documents. The W3C is a neutral standards body responsible for defining future of Internet. They do not support every new idea that comes along, and we should view their full support of XML (or any technology), as a harbinger of where tomorrow's Internet will take us. Ignore XML if you will, but know that it is most definitely a legitimate technology that will revolutionize way that we program applications for Web