Since its evolution, the HTML (Hyper Text Markup Language) has been revised and upgraded a number of times in order to make the World Wide Web a better place to live virtually. HTML is a derivative of SGML (Standard Generalized Markup Language), but its major flaw is that it is not extensible. Thus HTML is not appropriate for deployment under a network environment.
So, the World Wide Web Consortium (popularly known as W3C) decided to develop a new breed of extensible language and came up with XML, which stands for eXtensible Markup Language. Web developers use XML to dynamically describe the information stored on a web page. Though XML, the new generation web pages describes themselves which is helpful to gain good rankings in search engine listings and help the developers to write codes for custom search tools.
• XML shall be straightforwardly usable over the Internet.
• XML shall support a wide variety of applications.
• XML shall be compatible with SGML.
• It shall be easy to write programs that process XML documents.
• The number of optional features shall be kept to the absolute minimum, ideally zero.
• XML documents shall be human-legible and reasonably clear.
• The XML design should be prepared quickly.
• The design of XML shall be formal and concise.
• XML documents shall be easy to create.
• Terseness in XML markup is of minimal importance.
(Source: http://www.w3.org/TR/REC-xml/)
XML is referred as a Metalanguage which describes other languages. It is called so because XML allows the web developers to create documents which describes themselves to their reader. Though the XML tags look like the HTML ones, the main difference between the two is that the HTML documents do not tell the web browser how to display the web page. On the other hand, the XML documents are downloaded by the web browser along with a style sheet or Document Type definition (DTD) which guides the web browser on how to display the web page.
In its raw form, an XML document looks like the HTML and SGML documents, but all are functionally different. The main difference between the HTML and XML coding is that the XML tags are very rigid. For example; the HTML language has tags which have a closing and which don’t have a closing section. But in XML either all the tags must exist in pair or announce to the reader that a closing tag is not present.
For example: in HTML you can use <BR> tag without having to close it, but in XML you have to use it as <BR>…</BR> or simply as <BR/> which announces that there is no closing tag for BR.
Another prominent difference between the two languages is that the XML requires all the tag attribute values to be enclosed within quotation marks. For example:
<font color=red>……</font>, this is wrong in XML. It should be like <font color=”red”>…….</font>