XHTML Definition

XHTML (Extensible Hypertext Markup Language) is a markup language that combines the flexibility of HTML (Hypertext Markup Language) with the strict syntax and extensibility of XML (Extensible Markup Language). Created as a successor to HTML, XHTML is a W3C standard that aims to improve the structure, compatibility, and interoperability of web documents by enforcing stricter syntax rules.

Unlike HTML, which can sometimes display pages with minor errors, XHTML documents must follow XML standard rules to be processed correctly by web browsers. Adherence to XML rules means that documents are consistently rendered across different browsers and devices, enhancing the user experience. XHTML allows IT professionals and online businesses to create web content that is both robust and versatile, supporting a wide range of applications and services across various fields.

XHTML vs. HTML

Here is an overview of the differences between XHTML and HTML.

A Brief History of XHTML

XHTML emerged in the late 1990s as a reformulation of HTML 4.01 using XML 1.0. The World Wide Web Consortium (W3C) introduced XHTML 1.0 in 2000, aiming to combine the widespread use and familiarity of HTML with the strict syntax and extensibility of XML. This move was intended to promote more rigorous web development practices, ensuring documents were well-formed and adhered to stricter standards. The introduction of XHTML was a significant step towards a more structured and interoperable web, emphasizing the importance of document validity and consistency across multiple platforms and browsers.

However, the evolution of web standards and the advent of HTML5 shifted the momentum back towards HTML. HTML5, introduced in 2014, embraced many of the web development community's practical needs, such as native multimedia support, more semantic elements, and new form controls, without requiring the strict syntactical rules of XHTML.

Despite the initial enthusiasm for XHTML and its strict compliance with XML, the web development community largely favored the flexibility and simplicity of HTML5. Consequently, XHTML's popularity declined, and XHTML 2.0, which was under development, was eventually abandoned in favor of HTML5.

Today, while XHTML still has its uses in specific contexts where XML compatibility is required, HTML5 is the de facto standard for creating robust and interactive web pages.

Why Is XHTML Used?

XHTML is used for several key reasons, including:

• Strict syntax for cleaner code. XHTML enforces strict syntax rules, such as requiring all elements to be correctly nested, closed, and lowercase. This leads to cleaner, more error-free code that is easier to maintain and debug.
• Cross-device compatibility. The strict standards of XHTML help to ensure that documents are rendered more consistently across different browsers and devices. This is essential for web applications that need to function seamlessly on multiple platforms.
• Integration with XML applications. Because XHTML is an application of XML, it integrates well with other XML applications. This is particularly useful for web services, content management systems (CMS), and applications that use data from various sources.
• Future-proofing content. XHTML’s adherence to XML standards means that content is more likely to be forward-compatible with emerging web technologies. This is essential for long-term content strategy and archiving.
• Accessibility and internationalization. XHTML is better at supporting accessibility standards and internationalization thanks to its strict syntax and compatibility with various document parsing technologies. This makes it ideal for projects that require adherence to accessibility guidelines or that need to support multiple languages.
• Development discipline. The requirement for well-formed code encourages better coding practices among developers. This improves the quality of web pages in terms of robustness and reliability.

Elements of XHTML

As a reformulation of HTML 4.01 in XML, XHTML employs many of the elements of HTML. Still, these elements must be used according to XML's strict syntax rules.

Here's an overview of key XHTML elements:

Structural Elements

• html. The root element that defines an XHTML document.
• head. Contains meta-information about the document, such as its title and links to stylesheets.
• title. Specifies the title of the document, which appears in the browser's title bar or tab.
• body.  Contains the content of the document, such as text, images, and links.

Text Formatting Elements

• p. Defines a paragraph.
• br. Inserts a line break.
• h1 to h6. Defines headers, with h1 being the highest level and h6 the lowest.
• strong. Indicates strong emphasis on contents, typically displayed as bold text.
• em. Indicates emphasis that subtly changes the meaning of a sentence, typically displayed as italic text.

Hyperlink and Image Elements

• a. Defines a hyperlink, linking to another page or a location within the same page.
• img. Embeds an image into the document. Requires src (source) and alt (alternative text) attributes.

List Elements

• ul. Defines an unordered list.
• ol. Defines an ordered list.
• li. Defines a list item, used within either ul or ol tags.

Table Elements

• table. Defines a table.
• tr. Defines a row in a table.
• td. Defines a cell in a table.
• th. Defines a header cell in a table.

Form Elements

• form. Defines an HTML form for user input.
• input. Defines an input field within a form.
• textarea. Defines a multiline input field (text area).
• label. Defines a label for an input element.
• button. Defines a clickable button.

Scripting and Style Elements

• script. Places scripts, such as JavaScript, within the document.
• style. Contains style information for the document, typically CSS.

Other Important Elements

• meta. Provides metadata about the HTML document, such as character set, author, and viewport settings.
• link. Used to link external resources like CSS files to the document.
• div. Defines a division or a section in an HTML document and is used for styling or scripting purposes.
• span. Defines an inline container used to mark up a section of a text or document for styling or scripting.

Each of these elements must be used following XHTML's strict syntax rules, such as closing all tags (e.g., <p></p>) and using lowercase tags. In addition, attributes must be quoted, and elements like br and img must be self-closed if they do not contain any content (e.g., <br />, <img src="image.jpg" alt="Description" />).

XHTML Constraints

XHTML carries some constraints developers should keep in mind.

Mandatory XHTML <!DOCTYPE> Declaration

In XHTML, the mandatory <!DOCTYPE> declaration at the beginning of the document, before the <html> tag, defines the document type and version to the browser. This helps the browser to render the page correctly by informing it about the type of XHTML being used (Strict, Transitional, or Frameset).

Nested Elements

XHTML requires that elements be properly nested within each other, maintaining a clear and logical structure. This means that if an element is opened within another element, it must be closed before the outer element is closed. Proper nesting enables XML parsers to correctly interpret the document's structure, helping to prevent rendering issues across different browsers and devices.

XHTML Elements Must Be Closed

Unlike HTML, where some elements can be left open, and the browser will still render the content correctly, XHTML's adherence to XML syntax rules means that every start tag must have a corresponding end tag. Closing tags helps eliminate ambiguity in the document structure, ensuring that parsers can accurately interpret the content and structure of the document.

Empty Elements Must Be Closed

In XHTML, even empty elements, such as <br>, <img>, and <hr>, must still be closed. This is typically done using a self-closing tag syntax (e.g., <br /> or <img src="image.jpg" alt="Description" />). This rule reinforces the XML requirement that every element must be explicitly opened and closed, ensuring document structure clarity and aiding in error-free parsing by XML processors.

Elements Must Be in Lowercase

XHTML enforces the use of lowercase for all element names, reflecting its XML foundation, which is case-sensitive. This contrasts with HTML, where element names can be in uppercase, lowercase, or a mix of both. The requirement for lowercase element names in XHTML promotes consistency and reduces the likelihood of errors during document processing, making the code more readable and easier to manage.

Attribute Names Must Be in Lowercase

Like element names, all attribute names in XHTML must be in lowercase to be correctly interpreted. Ensuring that attribute names are in lowercase helps maintain consistency across the document, simplifies debugging, and enhances compatibility with XML tools and technologies.

Attribute Values Must Be Quoted

In XHTML, the values assigned to attributes must always be enclosed in quotes (either single ' or double " quotes). Quoting attributes ensures that their values are correctly interpreted by parsers, particularly when values contain spaces or special characters.

Attribute Minimization Is Forbidden

XHTML does not allow attribute minimization, a common practice in HTML, where an attribute does not need to be assigned a value. In XHTML, all attributes must be explicitly defined with a value, even if it means repeating the attribute name as its value. For example, a minimized HTML attribute like checked in XHTML must be written as checked="checked". This rule eliminates ambiguity and aligns with XML's criteria for well-formed documents.