HTML Versus XHTML

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HTML Versus XHTML

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XHTML is a stricter, more XML-based version of HTML.

What is XHTML?

XHTML stands for EXtensible HyperText Markup Language
XHTML is a stricter, more XML-based version of HTML
XHTML is HTML defined as an XML application
XHTML is supported by all major browsers

Why XHTML?

XML is a markup language where all documents must be marked up correctly (be "well-formed").

XHTML was developed to make HTML more extensible and flexible to work with other data formats (such as XML). In addition, browsers ignore errors in HTML pages, and try to display the website even if it has some errors in the markup. So XHTML comes with a much stricter error handling.

If you want to study XML, please read our XML Tutorial.

The Most Important Differences from HTML

<!DOCTYPE> is mandatory
The xmlns attribute in <html> is mandatory
<html>, <head>, <title>, and <body> are mandatory
Elements must always be properly nested
Elements must always be closed
Elements must always be in lowercase
Attribute names must always be in lowercase
Attribute values must always be quoted
Attribute minimization is forbidden

XHTML - <!DOCTYPE ....> Is Mandatory

An XHTML document must have an XHTML <!DOCTYPE> declaration.

The <html>, <head>, <title>, and <body> elements must also be present, and the xmlns attribute in <html> must specify the xml namespace for the document.

Example

Here is an XHTML document with a minimum of required tags:

<!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>

XHTML Elements Must be Properly Nested

In XHTML, elements must always be properly nested within each other, like this:

Correct:

Some text

Wrong:

Some text

XHTML Elements Must Always be Closed

In XHTML, elements must always be closed, like this:

Correct:

This is a paragraph
This is another paragraph

Wrong:

This is a paragraph
This is another paragraph

XHTML Empty Elements Must Always be Closed

In XHTML, empty elements must always be closed, like this:

Correct:

A break: 
A horizontal rule: <hr />
An image: <img src="happy.gif" alt="Happy face" />

Wrong:

A break: 
A horizontal rule: <hr>
An image: <img src="happy.gif" alt="Happy face">

XHTML Elements Must be in Lowercase

In XHTML, element names must always be in lowercase, like this:

Correct:

<body>
This is a paragraph
</body>

Wrong:

<BODY>
This is a paragraph
</BODY>

XHTML Attribute Names Must be in Lowercase

In XHTML, attribute names must always be in lowercase, like this:

Correct:

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

Wrong:

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

XHTML Attribute Values Must be Quoted

In XHTML, attribute values must always be quoted, like this:

Correct:

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

Wrong:

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

XHTML Attribute Minimization is Forbidden

In XHTML, attribute minimization is forbidden:

Correct:

Wrong:

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