Now, let’s explore the historical relationship and key differences between HTML and XHTML. Understanding this evolution provides valuable context for modern web development.

What is HTML? (A Quick Recap)

HTML (HyperText Markup Language) is the standard markup language for creating web pages. It describes the structure of a web page using a series of elements (tags) that tell the browser how to display content. It’s relatively forgiving; browsers try their best to render even poorly written HTML.

What is XHTML?

XHTML (eXtensible HyperText Markup Language) was a reformulation of HTML as an XML (eXtensible Markup Language) application. It aimed to combine the familiar structure of HTML with the strictness and extensibility of XML.

In essence, XHTML is HTML written with XML rules.

Why was XHTML Created? (The Vision)

In the late 1990s and early 2000s, there was a push for stricter, more standardized web content. The vision behind XHTML included:

  1. Purity and Strictness: HTML had become quite messy with inconsistent practices. XHTML aimed to enforce stricter syntax rules, making documents well-formed and easier for machines to parse.
  2. Extensibility: Being based on XML, XHTML offered potential for extending HTML with custom tags (though this didn’t widely materialize in practice).
  3. Portability: The idea was that strictly formed XHTML would be easier to parse and display on various devices, including early mobile phones and specialized internet appliances, not just traditional desktop browsers.
  4. Integration with XML Tools: Because it was XML, XHTML documents could be processed and transformed using standard XML tools.

Key Differences: HTML vs. XHTML (The Strict Rules)

The core difference lies in the syntax rules. XHTML enforces a much stricter set of rules compared to traditional HTML. Here are the most important ones:

  1. All Elements Must Be Properly Nested:
    • HTML (forgiving): <b><i>This is bold and italic.</b></i> (Incorrect nesting, but browsers often fix it)
    • XHTML (strict): <b><i>This is bold and italic.</i></b> (Must be <i> closed before <b>)
  2. All Elements Must Be Closed:
    • HTML (forgiving): <p>This is a paragraph. (Many browsers would assume </p>)
    • XHTML (strict): <p>This is a paragraph.</p> (All tags must have a closing tag)
  3. Empty Elements Must Be Self-Closed:
    • HTML (forgiving): <br>, <img>, <input>
    • XHTML (strict): <br />, <img src=”image.jpg” alt=”My Image” />, <input type=”text” name=”username” /> (Note the space before /> – this was important for backward compatibility with older HTML browsers that didn’t understand <tag/>)
  4. Attribute Values Must Be Quoted:
    • HTML (forgiving): <p align=center>
    • XHTML (strict): <p align=”center”> (Always use single or double quotes)
  5. Attribute Minimization is Forbidden:
    • HTML (forgiving): <input type=”checkbox” checked>
    • XHTML (strict): <input type=”checkbox” checked=”checked”> (Attribute and value must be explicitly paired)
  6. Element and Attribute Names Must Be Lowercase:
    • HTML (forgiving): <A HREF=”page.html”>Link</A>
    • XHTML (strict): <a href=”page.html”>Link</a> (Case-sensitive because XML is case-sensitive)
  7. Root Element (<html> tag):
    • XHTML documents require a <!DOCTYPE> declaration and a xmlns attribute within the <html> tag to specify the XML namespace.

XHTML Example:

XML

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
</html>

The Decline of XHTML (and the Rise of HTML5)

Despite its initial promise, XHTML never fully replaced HTML for general web development, primarily for these reasons:

  1. Browser Forgiveness: Web browsers continued to be very forgiving with HTML. Even if you wrote “bad” HTML, browsers would often try to display it, leading to less incentive for developers to adopt the strictness of XHTML.
  2. Learning Curve: The strict rules could be cumbersome for many developers.
  3. MIME Type Issue: For a browser to truly treat XHTML as XML, it needed to be served with the application/xhtml+xml MIME type. However, many browsers (especially older ones) only supported text/html. If served as application/xhtml+xml and there was a single syntax error, the browser would display an XML parsing error, not a partially rendered page, which was very user-unfriendly.
  4. HTML5’s Emergence: The development of HTML5 (which started around 2004-2007 and gained momentum) effectively superseded XHTML’s goals. HTML5 aimed to be:
    • Backward Compatible: It could be parsed by existing HTML browsers.
    • New Features: It introduced powerful new elements (like <video>, <audio>, <canvas>), APIs, and improved semantics.
    • Flexible Syntax: It retained the more forgiving syntax of traditional HTML, while also defining stricter parsing rules for browsers internally.

Modern Web Development: HTML5 is King

Today, when developers talk about HTML, they are almost universally referring to HTML5.

  • HTML5 embraces a more flexible syntax, allowing both “HTML-style” (e.g., <br>) and “XHTML-style” (e.g., <br />) empty tags, although the HTML-style is more common.
  • It focuses on creating robust and semantic web content.
  • It is the standard for building modern, interactive, and media-rich web applications.

The simple <!DOCTYPE html> declaration for HTML5 is a testament to its streamlined approach, replacing the complex XHTML DOCTYPES.

While XHTML was an interesting and influential step in the evolution of web standards, it has largely been replaced by HTML5. You will primarily be writing HTML5 for modern web development. Understanding XHTML’s principles helps appreciate the history and the shift towards HTML5’s pragmatism and new features.

Do It Yourself

Create a simple HTML form with a text input.
HTML

<form action="process_data.html" method="get">
    <label for="message">Your Message:</label>
    <input type="text" id="message" name="msg">
    <input type="submit" value="Send">
</form>

Preview

In the text input, type a phrase that includes spaces and special characters (e.g., “Hello & Goodbye!”).

Submit the form and observe the URL in your browser’s address bar. See how the spaces and & symbol have been URL encoded.

For HTML vs. XHTML:

Open your preferred text editor.Try writing a very simple HTML document, first adhering to strict XHTML rules (self-closing tags, lowercase, quoted attributes).

HTML

<!DOCTYPE html> <html lang="en">
<head>
    <meta charset="UTF-8" />
    <title>My Strict HTML Page</title>
</head>
<body>
    <p>This is a paragraph.</p>
    <img src="placeholder.png" alt="Placeholder" />
    <br />
</body>
</html>

Preview

My Strict HTML Page

This is a paragraph.

Placeholder

Then, write the same content using typical, more relaxed HTML5 syntax (e.g., omit self-closing slashes, use mixed case for tags).
HTML

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>My Relaxed HTML Page</title>
</head>
<body>
    <p>This is a paragraph.
    <img src="placeholder.png" alt="Placeholder">
    <br>
</body>
</html>

Preview

My Relaxed HTML Page

This is a paragraph. Placeholder

 (Note: Even in “relaxed” HTML5, it’s generally good practice to close <p> tags, but the point here is to highlight the historical differences in strictness).

Open both files in your browser. You’ll likely see no visual difference because modern browsers are designed to handle both. This exercise primarily helps you understand the historical context and the different approaches to syntax.