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HTML vs. XHTML: Understanding the Key Differences

HTML and XHTML are used to create web pages, they have key differences in syntax, structure, and purpose.

What is XHTML?

XHTML (Extensible Hypertext Markup Language) is a reformulation of HTML (Hypertext Markup Language) as an application of XML (Extensible Markup Language). It was designed to bring the structure and syntax of HTML into conformity with XML standards. XHTML is essentially an XML-compliant version of HTML, and it follows stricter rules than traditional HTML.

Why XHTML?

XHTML (Extensible Hypertext Markup Language) was introduced with the goal of combining the best features of HTML (Hypertext Markup Language) and XML (Extensible Markup Language) to create a more structured, extensible, and well-formed web markup language. Here are some reasons why XHTML was considered and used in the past:

The Most Important Differences from HTML

The most important differences between XHTML (Extensible Hypertext Markup Language) and HTML (Hypertext Markup Language) lie in their syntax, structure, and adherence to XML standards. Here are the key distinctions:

XML Syntax:

  • HTML: HTML has a more forgiving syntax, allowing for loosely closed tags and case-insensitive attribute names. It is not required to be well-formed XML.
  • XHTML: XHTML adheres to XML rules, requiring strict syntax, properly nested tags, lowercase tag names and attribute names, and attribute values enclosed in quotes. It must be well-formed XML.

Tag Closure:

  • HTML: In HTML, some tags, such as <br>, <img> , or <input>, don't necessarily need to be explicitly closed. It is common to use self-closing syntax for these tags (e.g., <br />, <img />, <input />).
  • XHTML: All tags in XHTML must be properly closed, either with a closing tag or with self-closing syntax. For example, <br />, <img />, and <input /> are used to close empty elements.

Document Structure:

  • HTML: HTML documents do not require a specific doctype declaration or XML namespace declaration.
  • XHTML: XHTML documents must begin with a specific doctype declaration, indicating the version of XHTML being used. They may also include an XML namespace declaration (xmlns) to specify the XML namespace.

Error Handling:

  • HTML: Browsers are generally forgiving of errors in HTML, and documents with syntax errors may still render. However, the rendering might be inconsistent.
  • XHTML: Browsers are less forgiving of errors in XHTML. If an XHTML document contains syntax errors, it may not render correctly or at all.

Compatibility:

  • HTML: HTML is widely supported by all modern browsers. Its more forgiving syntax allows for broader compatibility.
  • XHTML: Older browsers might not handle XHTML well due to its stricter syntax. While modern browsers can interpret both HTML and XHTML, the distinction has become less relevant with the widespread adoption of HTML5.

Use of XML Tools:

  • HTML: HTML is not designed to be processed with XML tools, and it may not be well-formed XML.
  • XHTML: XHTML can be processed with XML tools, making it suitable for environments where XML processing and validation are essential

XHTML - <!DOCTYPE ....> Is Mandatory

Yes, in XHTML, the <!DOCTYPE> declaration is mandatory. The <!DOCTYPE> declaration specifies the document type and version of XHTML that the document adheres to. It serves to inform the web browser or XML parser about the rules and specifications that should be applied when interpreting the document

Here is an example of a typical XHTML document with a <!DOCTYPE> declaration:

A Simple HTML Document

Example

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>XHTML Example</title>
</head>
<body>
    <h1>Hello, XHTML!</h1>
    <p>This is an example of an XHTML document.</p>
</body>
</html>
You can click on above box to edit the code and run again.

Output

XHTML Elements Must be Properly Nested

In XHTML, elements must always be properly nested within each other, like this:

Correct

<b><i>Some text</i></b>

Wrong

<b><i>Some text</b></i>