HTML vs. 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 (like XML).

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 Example

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
  <title>Title of document</title>
</head>
<body>
  <p>This is strict HTML.</p>
  <br />
</body>
</html>