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 is used?
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.
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.
<!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
Correct
<b><i>Some text</i></b>
Wrong
<b><i>Some text</b></i>
XHTML Elements Must Always be Closed
In XHTML, elements must always be closed
Correct
<p>This is a paragraph</p>
<p>This is another paragraph</p>
Wrong
<p>This is a paragraph
<p>This is another paragraph
XHTML Empty Elements Must Always be Closed
In XHTML, empty elements must always be closed
Correct
A break: <br />
A horizontal rule: <hr />
An image: <img src="happy.gif" alt="Happy face" />
Wrong
A break: <br>
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>
<p>This is a paragraph</p>
</body>
Wrong
<BODY>
<P>This is a paragraph</P>
</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>
What is HTML?
HTML stands for Hyper Text Markup Language
HTML is the standard markup language for creating Web pages
HTML describes the structure of a Web page
HTML consists of a series of elements
HTML elements tell the browser how to display the content
HTML elements label pieces of content such as "this is a heading", "this is a paragraph", "this is a link", etc.
A Simple HTML Document
<!DOCTYPE html>
<html>
<head>
<title>Page Title</title>
</head>
<body>
<h1>My First Heading</h1>
<p>My first paragraph.</p>
</body>
</html>
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