A gentle introduction to XML markup
Elements' attributes must always be quoted
XML element are often qualified using attributes. For example, an integer might be marked up as a length and the length element might be qualified to denote feet as the unit of measure. For example: <length unit='feet'>5</length>. The attribute is named unit, and it's value is always quoted. It does not matter whether or not it is quoted with an apostrophe (') or a double quote (").
There are only five entities defined by default
Certain characters in XML documents have special significance, specifically, the less than (<), greater than (>), and ampersand (&) characters. The first two characters are used to delimit the existence of ele- ment names. The ampersand is used to delimit the display of special characters commonly known as en- tities; they ampersand character is the "escape" character. Consequently, if you want to display any of these three characters in your XML documents, then you must express them in their entity form:
to display the & character type &
to display the < character type <
to display the > character type >
XML processors, computer programs that render XML documents, should be able interpret these char- acters without the characters being previously defined.
There are two other characters that can be represented as entity references:
to display the ' character optionally type '
to display the " character optionally type "
The concept of a "namespace" is used to avoid clashes in XML vocabularies.
Remember, the X in XML stands for extensible. This means you are allowed to create your own XML vocabulary. There is no centralized authority to dictate what all the valid vocabularies are and how they are used. Fine, but with so many XML vocabularies there are bound to be similarities between them. For example, it is quite likely that different vocabularies will want some sort of date element. Others will want a name or description element. In each of these vocabularies the values expected to be stored in date, name, or description elements may be different. How to tell the difference? Namespaces.
Namespaces have a one-to-one relationship with a URI (Universal Resource Identifier), and namespace attributes defined with URIs can be inserted into XML elements to denote how an element is to be used. Namespace attributes always begin with "xmlns". Namespace attributes are always end with some sort of identifier call the "local part". These two things are always delimited by a colon and finally equated with a URI. For example, a conventional namespace defining the Dublin Core namespace is written as: