Core Code
Core Code Modern Dev Handbook

HTML vs. XHTML

XHTML is a stricter, XML-based version of HTML that enforces well-formed and error-free markup. Unlike HTML, XHTML requires lowercase tags, quoted attributes, and properly closed elements.
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HTML Versus XHTML

XHTML is a stricter, XML-based version of HTML that enforces cleaner and more consistent markup rules.

What Is XHTML?

XHTML stands for Extensible HyperText Markup Language.

  • XHTML is HTML rewritten as an XML application
  • It follows strict XML syntax rules
  • All major web browsers support XHTML

The main goal of XHTML is to make web documents more structured, predictable, and compatible with XML-based systems.

Why XHTML?

XML requires documents to be well-formed, meaning that every element must follow strict syntax rules.

HTML browsers often ignore markup errors and still display the page, but XHTML enforces strict error handling. This helps developers write cleaner and more reliable code.

XHTML was designed to make HTML more extensible and easier to integrate with other data formats such as XML.

Key Differences Between HTML and XHTML

  • <!DOCTYPE> declaration is mandatory
  • The xmlns attribute is required in the <html> element
  • <html>, <head>, <title>, and <body> are required
  • Elements must be properly nested
  • All elements must be closed
  • Element and attribute names must be lowercase
  • Attribute values must always be quoted
  • Attribute minimization is not allowed

XHTML Doctype and Document Structure

An XHTML document must include a valid XHTML doctype and define the XML namespace.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>Title of document</title> </head> <body> some content here... </body> </html>

Proper Nesting of Elements

In XHTML, elements must always be nested correctly.

Correct:

<b><i>Some text</i></b>

Incorrect:

<b><i>Some text</b></i>

Closing Elements

All XHTML elements must be explicitly closed.

Correct:

<p>This is a paragraph</p>

Incorrect:

<p>This is a paragraph

Empty Elements Must Be Closed

Empty elements must include a closing slash.

Correct:

<br /> <hr /> <img src="happy.gif" alt="Happy face" />

Incorrect:

<br> <hr> <img src="happy.gif" alt="Happy face">

Lowercase Element and Attribute Names

All element and attribute names must be written in lowercase.

Correct:

<a href="https://www.w3schools.com/html/">Visit our HTML tutorial</a>

Incorrect:

<a HREF="https://www.w3schools.com/html/">Visit our HTML tutorial</a>

Quoted Attribute Values

All attribute values must be enclosed in quotes.

Correct:

<input type="checkbox" checked="checked" />

Incorrect:

<input type="checkbox" checked />

Summary

XHTML enforces strict syntax rules to ensure clean, well-structured documents.

While HTML is more forgiving, XHTML is ideal for developers who want predictable markup and better integration with XML-based technologies.