Francisco S.| Rules for XHTML

What Is XHTML?

XHTML stands for EXtensible HyperText Markup Language
XHTML is aimed to replace HTML
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 a W3C Recommendation

The Most Important Differences:

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 Elements Must Be In Lower Case

The XHTML specification defines that the tag names and attributes need to be lower case.

XHTML Documents Must Have One Root Element

All XHTML elements must be nested within the <html> root element. All other elements can have sub (children) elements. Sub elements must be in pairs and correctly nested within their parent element.

Some More XHTML Syntax Rules:

Attribute names must be in lower case
Attribute values must be quoted
Attribute minimization is forbidden
The id attribute replaces the name attribute
The XHTML DTD defines mandatory elements

Mandatory XHTML Elements

All XHTML documents must have a DOCTYPE declaration. The html, head and body elements must be present, and the title must be present inside the head element.

<!DOCTYPE> Is Mandatory

An XHTML document consists of three main parts:

the DOCTYPE
the Head
the Body

The 3 Document Type Definitions

DTD specifies the syntax of a web page in SGML.
DTD is used by SGML applications, such as HTML, to specify rules that apply to the markup of documents of a particular type, including a set of element and entity declarations.
XHTML is specified in an SGML document type definition or 'DTD'.
An XHTML DTD describes in precise, computer-readable language, the allowed syntax and grammar of XHTML markup.
There are currently 3 XHTML document types:

STRICT
TRANSITIONAL
FRAMESET

Empty Tags: <hr> , and <img>

Empty tags are not allowed in XHTML. The <hr> and tags should be replaced with <hr /> and .

This produced a problem with Netscape that misinterpreted the tag. We don't know why, but changing it to worked fine. After that discovery, a general search and replace function was executed to swap the tags.

A few other tags (like the <img> tag) were suffering from the same problem as above. We decided not to close the <img> tags with </img>, but with /> at the end of the tag. This was done manually.

Validate XHTML With A DTD

An XHTML document is validated against a Document Type Definition (DTD). Before an XHTML file can be properly validated, a correct DTD must be added as the first line of the file.

The Strict DTD includes elements and attributes that have not been deprecated or do not appear in framesets:

!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"
The Transitional DTD includes everything in the strict DTD plus deprecated elements and attributes:

!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"
The Frameset DTD includes everything in the transitional DTD plus frames as well:

!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Frameset//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd"

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