<!DOCTYPE> Is Mandatory
An XHTML document consists of three main parts:
- the DOCTYPE declaration
- the <head> section
- the <body> section
The basic document structure is:
<!DOCTYPE ...>
<html>
<head>
<title>... </title>
</head>
<body> ... </body>
</html>
<html>
<head>
<title>... </title>
</head>
<body> ... </body>
</html>
Note: The <!DOCTYPE> declaration refers to a Document Type Definition (DTD). A DTD specifies the rules for the markup language, so that the browsers render the content correctly.
An XHTML Example
The example below 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>
</body>
</html>
"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>
</body>
</html>
Note: The xmlns attribute in <html>, specifies the xml namespace for a document, and is required in XHTML documents.
XHTML Different Doctypes
The <!DOCTYPE> declaration is the very first thing in an XHTML document, before the <html> tag.
The <!DOCTYPE> declaration is not an XHTML tag; it is an instruction to the web browser about what version of the markup language the page is written in.
The <!DOCTYPE> declaration refers to a Document Type Definition (DTD). The DTD specifies the rules for the markup language, so that the browsers render the content correctly.
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">
XHTML HowTo
The following steps shows how a website can be converted from HTML to XHTML in 6 simple steps:
1. Add a <!DOCTYPE>
Add an XHTML <!DOCTYPE> to the first line of every page:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
Tip: Your pages must have a DOCTYPE declaration if you want them to validate as XHTML.
2. Add an xmlns Attribute
Add an xmlns attribute to the html element of every page:
<html xmlns="http://www.w3.org/1999/xhtml">
Note: The xmlns attribute specifies the xml namespace for a document, and is required in XHTML documents.
3. Change Tags And Attribute Names to Lowercase
A general "find-and-replace" function was executed to replace all uppercase tags with lowercase tags. The same was done for attribute names.
4. Quote All Attribute Values
Check every page to see that attribute values are quoted.
5. Close all Empty Tags
Empty tags are not allowed in XHTML. The <hr> and <br> tags should be replaced with <hr /> and <br />.
A general "find-and-replace" function was executed to swap the tags.
Close the <img> tags with /> the end of the tag.
6. Validate XHTML With The W3C Validator
Before an XHTML file can be validated, a correct DTD must be added as the first line of the file.
Correct errors found (if any).
XHTML Quiz Test
The test contains 20 questions and there is no time limit.
The test is not official, it's just a nice way to see how much you know, or don't know, about XHTML.
You will get 1 point for each correct answer. At the end of the Quiz, your total score will be displayed. Maximum score is 20 points.
Good luck! Start the XHTML Quiz
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