PPT Slide
Structure of XML and well formed data
Extends your ability to describe a document. Html restricts the use of which tags and vocabulary you provide, whereas XML enables you to define meaningful tags for your applications. For example you might wish to define a Book in XML, which comprises of a Title, author, ISBN number, price and number of pages. All of these can be defined into a number of tags which comprise a catalogue of books. Whilst this can be slightly disconcerting to start with (not having a reference book handy with all your tags neatly defined - as in html), it provides the developer with more power to structure data more thoroughly - and enables you to enforce structural rules (as we will see with the use of a document type definition later). Remember however - you are extending your tags to identify elements by what they are, not by how they look.
This is essential for documents to make sense. Without markup your computer views your document as one long string of text, with each character having equal importance to every other character. By ‘marking up’ your documents, you provide meaning to the pieces within, you identify them in a way that gives them value and context - this is book, this is title, this is a section head, etc.
A language describes something by providing a set of rules. The developer is provided with the flexibility to create a user-defined set of markup tags, but the current XML structure and syntax of the language remain firm and clearly defined.