XHTML is HTML written as XML.
What Is XHTML?
- XHTML stands for EXtensible HyperText Markup Language
- XHTML is almost identical to HTML
- XHTML is stricter than 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.
This HTML code works fine in most browsers (even if it does not follow the HTML rules):
<html> <head> <title>This is bad HTML</title> <body> <h1>Bad HTML <p>This is a paragraph </body>
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 “bad” markup.
XML is a markup language where documents must be marked up correctly (be “well-formed”).
If you want to study XML, please read XML tutorial.
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 xmlns attribute in <html> is mandatory
- <html>, <head>, <title>, and <body> are 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.
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.
This example shows an XHTML document with a minimum of required tags:
<!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> some content </body> </html>
XHTML Elements Must Be Properly Nested
In HTML, some elements can be improperly nested within each other, like this:
<b><i>This text is bold and italic</b></i>
In XHTML, all elements must be properly nested within each other, like this:
<b><i>This text is bold and italic</i></b>
XHTML Elements Must Always Be Closed
This is wrong:
<p>This is a paragraph
<p>This is another paragraph
This is correct:
<p>This is a paragraph</p>
<p>This is another paragraph</p>
Empty Elements Must Also Be Closed
This is wrong:
A break: <br>
A horizontal rule: <hr>
An image: <img src=”happy.gif” alt=”Happy face”>
This is correct:
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:
<BODY>
<P>This is a paragraph</P>
</BODY>
This is correct:
<body>
<p>This is a paragraph</p>
</body>
XHTML Attribute Names Must Be In Lower Case
This is wrong:
<table WIDTH=”100%”>
This is correct:
<table width=”100%”>
Attribute Values Must Be Quoted
This is wrong:
<table width=100%>
This is correct:
<table width=”100%”>
Attribute Minimization Is Forbidden
Wrong:
<input type=”checkbox” name=”vehicle” value=”car” checked />
Correct:
<input type=”checkbox” name=”vehicle” value=”car” checked=”checked” />
Wrong:
<input type=”text” name=”lastname” disabled />
Correct:
<input type=”text” name=”lastname” disabled=”disabled” />
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
Validate XHTML With The W3C Validator
Go to:
http://validator.w3.org/
HTML <!DOCTYPE> Declaration
<!DOCTYPE html> <html> <head> <title>Title of the document</title> </head> <body> The content of the document...... </body> </html>
Definition and Usage
The <!DOCTYPE> declaration must be the very first thing in your HTML document, before the <html> tag.
The <!DOCTYPE> declaration is not an HTML tag; it is an instruction to the web browser about what version of HTML the page is written in.
In HTML 4.01, the <!DOCTYPE> declaration refers to a DTD, because HTML 4.01 was based on SGML. The DTD specifies the rules for the markup language, so that the browsers render the content correctly.
HTML5 is not based on SGML, and therefore does not require a reference to a DTD.
Tip: Always add the <!DOCTYPE> declaration to your HTML documents, so that the browser knows what type of document to expect.
Differences Between HTML 4.01 and HTML5
There are three different <!DOCTYPE> declarations in HTML 4.01. In HTML5 there is only one:
<!DOCTYPE html>
Tips and Notes
Tip: The <!DOCTYPE> declaration is NOT case sensitive.
Common DOCTYPE Declarations
HTML 5
<!DOCTYPE html>
HTML 4.01 Strict
This DTD contains all HTML elements and attributes, but does NOT INCLUDE presentational or deprecated elements (like font). Framesets are not allowed.
<!DOCTYPE HTML PUBLIC “-//W3C//DTD HTML 4.01//EN” “http://www.w3.org/TR/html4/strict.dtd”>
HTML 4.01 Transitional
This DTD contains all HTML elements and attributes, INCLUDING presentational and deprecated elements (like font). Framesets are not allowed.
<!DOCTYPE HTML PUBLIC “-//W3C//DTD HTML 4.01 Transitional//EN” “http://www.w3.org/TR/html4/loose.dtd”>
HTML 4.01 Frameset
This DTD is equal to HTML 4.01 Transitional, but allows the use of frameset content.
<!DOCTYPE HTML PUBLIC “-//W3C//DTD HTML 4.01 Frameset//EN” “http://www.w3.org/TR/html4/frameset.dtd”>
XHTML 1.0 Strict
This DTD contains all HTML elements and attributes, but does NOT INCLUDE presentational or deprecated elements (like font). Framesets are not allowed. The markup must also be written as well-formed XML.
<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Strict//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd”>
XHTML 1.0 Transitional
This DTD contains all HTML elements and attributes, INCLUDING presentational and deprecated elements (like font). Framesets are not allowed. The markup must also be written as well-formed XML.
<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Transitional//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd”>
XHTML 1.0 Frameset
This DTD is equal to XHTML 1.0 Transitional, but allows the use of frameset content.
<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.0 Frameset//EN” “http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd”>
XHTML 1.1
This DTD is equal to XHTML 1.0 Strict, but allows you to add modules (for example to provide ruby support for East-Asian languages).
<!DOCTYPE html PUBLIC “-//W3C//DTD XHTML 1.1//EN” “http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd”>
Source: w3schools.com