Element tags are the fundamental building blocks of XML documents. They define the structure and meaning of the data within the document. Here's a comprehensive explanation:

Core Concepts

• Structure and Meaning: Element tags provide structure by organizing data into a hierarchical tree, and they give meaning to the data by labeling it with descriptive names.

• User-Defined (Extensible): In XML, you, the document author, define the element tags. Unlike HTML, which has a fixed set of tags, XML allows you to create tags that are relevant to your specific data. This is what makes XML "extensible."

• Case-Sensitive: XML element tags are case-sensitive. <Book> is different from <book> and <bOOk>. This is a very important distinction from HTML.

• Nesting: Elements can be nested within other elements, creating parent-child relationships. This nesting forms the hierarchical structure of the document.

• Start and End Tags: Most elements have a start tag (<elementName>) and a corresponding end tag (</elementName>). The content of the element goes between these tags.

• Empty Elements: Elements that don't have any content can be represented in two ways:

• <elementName></elementName> (start tag immediately followed by end tag)

• <elementName /> (self-closing tag – a more concise way to represent an empty element)

• Root Element: Every XML document must have exactly one root element that encloses all other elements.

Syntax

• Start Tag: <tagName>

• End Tag: </tagName>

• Self-Closing Tag (for empty elements): <tagName />

• Content: The data that goes between the start and end tags. This content can be:

• Text (e.g., <title>The Lord of the Rings</title>)

• Other elements (nested elements)

• A mixture of text and other elements

• Nothing (empty element)

Rules for Element Tag Names

• Must Start with a Letter or Underscore: Tag names cannot start with a number or punctuation character (except underscore).

• Can Contain:

• Letters

• Digits

• Hyphens (-)

• Underscores (_)

• Periods (.)

• Cannot Contain:

• Spaces

• Most punctuation characters (except hyphen, underscore, and period)

• Cannot Start with "xml" (in any case combination): Names starting with "xml" (or "XML", "Xml", etc.) are reserved.

• Best Practices:

• Use descriptive and meaningful names.

• Use consistent naming conventions (e.g., lowercase, camelCase, snake_case). Be consistent within a document.

• Avoid using hyphens in element names, and use underscores instead. (This is not a strict rule, but often considered better)

Examples

• Simple Element:

XML

<title>The Hitchhiker's Guide to the Galaxy</title>

• title: Element tag name.

• <title>: Start tag.

• The Hitchhiker's Guide to the Galaxy: Text content.

• </title>: End tag.

• Nested Elements:

XML

<book>

    <title>Pride and Prejudice</title>

    <author>Jane Austen</author>

    <year>1813</year>

</book>

• book: The parent element.

• title, author, year: Child elements nested within book.

• Empty Element:

XML

<hr /> 

• hr: Element tag name (representing a horizontal rule, similar to HTML).

• <hr />: Self-closing tag (no content).

• Element with Attributes:

XML

<book category="fiction">

    <title>1984</title>

</book>

• category : Attribute name.

• "fiction" : Attribute value.

Well-Formedness Rules (Related to Tags)

These rules must be followed for an XML document to be considered well-formed:

• Matching Start and End Tags: Every start tag must have a corresponding end tag (unless it's a self-closing tag).

• Proper Nesting: Elements must be properly nested. This means an element that starts inside another element must also end inside that element. Overlapping tags are not allowed.

• Correct: <outer><inner>Content</inner></outer>

• Incorrect (Overlapping): <outer><inner>Content</outer></inner>

• One Root Element: The document must have exactly one root element that contains all other elements.

Distinction from HTML Tags

• Predefined vs. User-Defined: HTML has a fixed set of predefined tags (e.g., <h1>, <p>, <img>). You cannot create your own HTML tags. XML allows you to create any tags you need, as long as they follow the naming rules.

• Case Sensitivity: HTML tags are generally not case-sensitive (<P> is the same as <p>). XML tags are strictly case-sensitive (<Book> is different from <book>).

• Closing Tags: In HTML, some tags (e.g., <br>, <img>) don't require closing tags. In XML, all elements must either have a closing tag or be self-closing.

• Strictness: HTML is more forgiving of errors. XML requires strict adherence to well-formedness rules.

In essence, element tags in XML are the fundamental tools for structuring and labeling data. Their extensibility, combined with the strict rules of XML, makes the language powerful and reliable for representing a wide variety of information. The rules ensure consistency and allow for easy parsing and processing by computers