XML Introduction

Problems with HTML :

1.HTML has syntactic checking and Validation constraints:

There are formal definitions of the structure of HTML documents. HTML is an SGML application and there is a document type definition (DTD) for every version of HTML. Web browsers are designed to accept almost anything that looks even slightly like HTML .The only tag that is compulsory in an HTML document is the TITLE tag; and this is one of the least common tags there is.

2. HTML content awareness problems:

Searching the Web is complicated by the fact that HTML doesn’t give you a way to describe the information content i.e the semantics of documents. In XML you can use any tags you like (such as &ltNAME> instead of &ltH3>), but using attributes in tags (such as &ltH3 CLASS=“name”>) can embed just as much semantic information as custom tags can.

Without any agreement on tag names, the value of custom tags becomes a bit doubtful. To worsen matters, the same tag name in one context can mean something completely different in another. Furthermore, there are the complications of foreign languages—seeing &ltinkoopprijs> isn’t going to help very much if you don’t know that it’s Dutch for “purchase price.”

HTML is not object-oriented:

Modern programmers have been making a long and difficult transition to object-oriented techniques. They want to leverage these skills and have such things as inheritance, and HTML has done very little to accommodate them.

HTML lacks a robust linking mechanism:

If you’ve spent a few hours on the Web, you’ve probably encountered at least one broken link. HTML’s links are one-to-one, with the linking hard coded in the source HTML files. If the location of one target file changes, a Webmaster may have to update dozens or even hundreds of other pages.

HTML is not reusable:

Depending on how well written they are, HTML pages and fragments of HTML code can be extremely difficult to reuse because they are so specifically tailored to their place in the web of associated pages.

The Standard Generalized Markup Language (SGML) from which XML is derived, is useful to make data storage independent of any one software package or software vendor. SGML is a meta language, or a language for describing markup languages. HTML is one such markup language and is therefore called an SGML application. In XML, these applications are often called markup languages—such as the hand-held device markup language (HDML) and the FAQ markup language (QML).

But SGML is just too expensive and complicated for Web use on a large scale. Using SGML requires too much of an investment in time, tools, and training.

XML uses the features of SGML that it needs and tries to incorporate the lessons learned from HTML.

Advantages of XML :

1.XML can be used with existing Web protocols and mechanisms and it does not impose any additional requirements. XML has been developed with the Web in mind—features of SGML that were too difficult to use on the Web were left out, and features that are needed for Web use either have been added or are inherited from applications that already work.

2.XML supports a wide variety of applications. It is difficult to support a lot of applications with just HTML; hence, the growth of scripting languages. HTML is simply too specific. XML adopts the generic nature of SGML, but adds flexibility to make it truly extensible.

3. It is easy to write programs that process XML documents. One of the major strengths of HTML is that it’s easy for even a non-programmer to throw together a few lines of scripting code that enable you to do basic processing . HTML even includes some features of its own that enable you to carry out some basic processing .

4.XML documents are reasonably clear to the any one.A valid XML document
  1. Describes the structural rules that the markup attempts to follow
  2. Lists any external resources (external entities) that are part of the document
  3. Declares any internal resources (internal entities) that are used within the document
  4. Lists the types of non-XML resources (notations) used and identifies any helper applications that might be needed
  5. Lists any non-XML resources (binaries) that are used within the document and identifies any helper applications that might be needed
5.XML documents are easy to create. HTML is almost famous for its ease of use, and XML capitalizes on this strength.

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