` * _Note: empty elements can still have attributes_ * Tags are case sensitive * Attribute values must be quoted

### Intro XML tags are not defined in any XML standard, they are defined by the author/developer of the XML document. Has tree structure, elements/tags can be root, parent, child, or siblings. All elements can have text content and attributes. XML Prolog - `` , specifies character encoding to avoid unique characters like international characters. ### Some characterIstics of XML: * Must have a closing tag * We can however use a self-closing tag for empty elements - `` * _Note: empty elements can still have attributes_ * Tags are case sensitive * Attribute values must be quoted - `<title lang="en">example` * Certain characters should be replaced with safe characters. * Character like `<` will result in an error as the parser interprets it as the start of a new element * To avoid these error, there are replacement safe value chracter \(Entity references\), which are given below: * `<` = `<` less than * `>` = `>` greater than * `&` = `&` ampersand * `'` = `'` apostrophe * `"` = `"` quotation mark * white space is preserved - no truncation for multiple white-spaces * `` but no double dashes allow in middle `like -- this` * Newlines are stored as `LF` * For reference on how OS stores newlines them \(Carriage return & line feed\): * Windows - `CR + LF` * Unix & Mac OSX - `LF` * Old mac - `CR` * XML - `LF` * Elements naming rules: * must start with a letter or underscore \(except letters `xml` \) * case-sensitive * can contain letters, digits, hyphens, underscores, and periods * cannot contain spaces > **Tip:** An entity has three parts: it starts with an ampersand \(&\), then comes the entity name, and it ends with a semicolon \(;\). ### Namespaces XML Namespaces help resolve the element name conflicts. We can use prefix on element names to avoid the same name conflicts. However, a namespace for the prefix should be defined. `xmlns` attribute can be used to define a namespace. This can be declared in the attribute of the start tag of an element or on the XML root element itself. The namespace declaration has syntax `xmlns:prefix="URI"` . ```text ... or example 2 ``` Note: the URI will not be used by the parser to look up information but to give namespace a unique name. However, it is a good practice to have URI to a page explaining the namespace information for developers. **Default namespace** - We can also define a default namespace for an element to avoid using prefixes in all of its child elements. syntax = `xmlns="namespaceURI"` example: ```text example 2 ``` ### XML Validation There are two terms used in XML Validation: 1. "well formed" - A XML document with correct syntax 2. "valid" - A XML document that is "well formed" XML document and validated against a _**document type definition**_ > **Document type definition \(Schema of XML\)** - defines the rules, structure, standard, and allowed elements and attributes for an XML Document. There is two document type definition that can be used with XML: 1. [DTD ](https://www.w3schools.com/xml/xml_dtd.asp)- The original Document Type Definition 2. [XML Schema](https://www.w3schools.com/xml/xml_schema.asp) - An XML-based alternative to DTD