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HTML and XHTML
XHTML is HTML written as XML.
What Is XHTML?
XHTML stands for EXtensible HyperText Markup Language
XHTML is almost identical to HTML 4.01
XHTML is a stricter and cleaner version of HTML
XHTML is HTML defined as an XML application
XHTML is supported by all major browsers.
Why XHTML?
Many pages on the internet contain "bad" HTML.
The following HTML code will work fine if you view it in a browser.
Code:
<html>
<head>
<title>This is bad HTML</title>
<body>
<h1>Bad HTML
<p>This is a paragraph
</body>
XML is a markup language where documents must be marked up correctly and "well-formed".
Today's market consists of different browser technologies. Some browsers run on computers, and some browsers run on mobile phones or other small devices. Smaller devices often lack the resources or power to interpret a "bad" markup language.
Therefore - by combining the strengths of HTML and XML, XHTML was developed. XHTML is HTML redesigned as XML
The Most Important Differences from HTML:
Document Structure
XHTML DOCTYPE is mandatory
The XML namespace attribute in <html> is mandatory
<html>, <head>, <title>, and <body> is mandatory
XHTML Elements
XHTML elements must be properly nested
XHTML elements must always be closed
XHTML elements must be in lowercase
XHTML documents must have one root element
XHTML Attributes
Attribute names must be in lower case
Attribute values must be quoted
Attribute minimization is forbidden
<!DOCTYPE ....> Is Mandatory
An XHTML document must have an XHTML DOCTYPE declaration.
A complete list of all the XHTML Doctypes is found in our HTML Tags Reference.
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.
Code:
<html xmlns="http://www.bizhat.com/1999/xhtml">
<head>
<title>Title of document</title>
</head>
<body>
......
</body>
</html>
XHTML Elements Must Be Properly Nested
In HTML, some elements can be improperly nested within each other, like this:
Code:
<b><i>This text is bold and italic</b></i>
In XHTML, all elements must be properly nested within each other, like this:
Code:
<b><i>This text is bold and italic</i></b>
XHTML Elements Must Always Be Closed
This is wrong:
Code:
<p>This is a paragraph
<p>This is another paragraph
This is correct:
Code:
<p>This is a paragraph</p>
<p>This is another paragraph</p>
Empty Elements Must Also Be Closed
This is wrong:
Code:
A break: <br>
A horizontal rule: <hr>
An image: <img src="happy.gif" alt="Happy face">
This is correct:
Code:
A break: <br />
A horizontal rule: <hr />
An image: <img src="happy.gif" alt="Happy face" />
XHTML Elements Must Be In Lower Case
This is wrong:
Code:
<BODY>
<P>This is a paragraph</P>
</BODY>
This is correct:
Code:
<body>
<p>This is a paragraph</p>
</body>
Attribute Names Must Be In Lower Case
This is wrong:
Code:
<table WIDTH="100%">
This is correct:
Code:
<table width="100%">
Attribute Values Must Be Quoted
This is wrong:
This is correct:
Code:
<table width="100%">
Attribute Minimization Is Forbidden
This is wrong:
Code:
<input checked>
<input readonly>
<input disabled>
<option selected>
This is correct:
Code:
<input checked="checked">
<input readonly="readonly">
<input disabled="disabled">
<option selected="selected">
How to Convert from HTML to XHTML
Add an XHTML <!DOCTYPE> to the first line of every page
Add an xmlns attribute to the html element of every page
Change all element names to lowercase
Close all empty elements
Change all attribute names to lowercase
Quote all attribute values
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