15. XML (eXtensible Markup Language)

CSS Optional — Paper-II, Section-B · P-II.VI.15 · CSS 2018 asked to differentiate XHTML and XML. XML also appears in the Database section (XML documents & web services); this page focuses on the web-development angle.

1. Definition

• XML is a W3C markup language (Recommendation 1998), derived from SGML, for storing and transporting data in a format that is both human-readable and machine-readable.
• Extensible: there are no predefined tags — you define your own tags that describe the data (self-describing).
• XML carries data; HTML displays data.

2. Example

<?xml version="1.0" encoding="UTF-8"?>
<!-- Library catalogue -->
<library>
  <book id="B101" category="CS">
    <title>Web Engineering</title>
    <author>Kappel</author>
    <year>2006</year>
    <price currency="PKR">4500</price>
  </book>
  <book id="B102" category="CS">
    <title>Digital Image Processing</title>
    <author>Gonzalez</author>
    <year>2018</year>
    <price currency="PKR">6000</price>
  </book>
</library>

3. Rules for Well-Formed XML

1. Exactly one root element.
2. Every start tag has a matching end tag (or is self-closing: ``).
3. Tags are case-sensitive (<Book> ≠ <book>).
4. Elements properly nested (no overlapping).
5. Attribute values always quoted.
6. Special characters escaped (< > & ' ").
7. The XML declaration, if present, must be first.

Well-formed vs valid: well-formed follows the syntax rules; valid is well-formed and conforms to a schema (DTD or XSD).

4. DTD and XML Schema (XSD)

<!-- DTD -->
<!DOCTYPE library [
  <!ELEMENT library (book+)>
  <!ELEMENT book (title, author, year, price)>
  <!ATTLIST book id ID #REQUIRED>
  <!ELEMENT title (#PCDATA)>
]>

<!-- XSD fragment -->
<xs:element name="year" type="xs:integer"/>
<xs:element name="price" type="xs:decimal"/>

5. The XML Family

• Namespaces: avoid name clashes (xmlns:h="...").
• XPath: navigate/select nodes (/library/book[price>5000]/title).
• XSLT: transform XML into HTML or other XML.
• XQuery: query XML collections.
• Parsers: DOM (tree in memory) and SAX (event-based streaming).

6. Uses of XML

Data exchange between systems; configuration files (e.g. Android manifests, Java/Struts configs); RSS/Atom feeds; SOAP web services and WSDL; SVG graphics; XHTML; office formats (DOCX, XLSX are zipped XML); sitemaps (sitemap.xml) for SEO.

7. Comparisons

XML vs HTML

XHTML vs XML (CSS 2018)

XML vs JSON

Same data in JSON: {"book": {"id": "B101", "title": "Web Engineering", "year": 2006}}

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🎯 CSS Exam Angle

Define XML → show a small example → well-formedness rules → DTD/XSD → uses. For comparison questions (XHTML vs XML, XML vs HTML/JSON) the tables above are complete answers; add the one-line relationship "every XHTML document is XML".

📌 Past CSS Paper Frequency

⚠️ MCQ Traps

• XML tags are case-sensitive and user-defined.
• Well-formed ≠ valid: valid also conforms to a DTD/XSD.
• XSD is written in XML; DTD is not.
• XSLT transforms XML; XPath selects nodes.
• XML was designed to carry data, not display it.

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✅ Self-Check (with Answers)

Q1. Find the errors: <Student><name>Ali</Name><age>21<age></Student><Student>...</Student>

Ans: Case mismatch (name vs Name); age not closed properly (needs </age>); two root elements — wrap them in a single root such as <students>.

Q2. Differentiate well-formed and valid XML.

Ans: Well-formed XML follows syntax rules (one root, closed and properly nested tags, quoted attributes, case consistency). Valid XML is well-formed and also conforms to a DTD or XML Schema.

Q3. Write an XPath to select titles of books cheaper than 5000.

Ans: /library/book[price < 5000]/title

Q4. Why have JSON APIs largely replaced XML in web apps?

Ans: JSON is more compact (less bandwidth), parses natively into JavaScript objects, and supports basic data types directly; XML remains useful where schemas, namespaces, documents or legacy SOAP are required.

Q5. Is every XML document an XHTML document?

Ans: No. XHTML is one particular XML vocabulary with fixed HTML tags; XML can define any tags. Every XHTML document is XML, but not vice versa.

📜 Past-Paper Questions & MCQs (CSS 2016–2026)

2018 · Paper-II · Q8(B) · 6 marks

Q. Differentiate between XHTML and XML.

Ans.

2022 · Paper-II · Q8(b) · XML part

Q. Discuss XML.

Ans. XML is a text-based, self-describing markup language with user-defined tags. It stores data in a hierarchical (tree) structure, is platform independent, and is used for data exchange, configuration files and web services. A document must be well-formed (one root element, properly nested and closed tags), and can be validated against a DTD or schema.

<student><name>Ali</name><regNo>101</regNo></student>