HTML vs XHTML

Understanding the differences between HTML and XHTML and when to use each

XHTML (Extensible HyperText Markup Language) is a stricter, XML-based version of HTML. While modern web development has largely moved to HTML5, understanding the differences between HTML and XHTML is important for maintaining legacy code and writing well-formed markup.

What is XHTML?

XHTML is HTML written as XML. It combines the flexibility of HTML with the strict syntax rules of XML.

  • XHTML 1.0: Released in 2000, reformulated HTML 4.01 as XML
  • XHTML 1.1: Released in 2001, modularized version
  • XHTML5: XML serialization of HTML5 (rarely used today)

Key Point

Modern web development uses HTML5. XHTML is primarily important for understanding legacy code and maintaining existing XHTML websites.

Key Differences Between HTML and XHTML

1. Document Structure

HTML5 Document

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>My Page</title>
</head>
<body>
    <h1>Welcome</h1>
</body>
</html>

XHTML Document

<!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" xml:lang="en" lang="en">
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    <title>My Page</title>
</head>
<body>
    <h1>Welcome</h1>
</body>
</html>

2. Element and Attribute Rules

Rule HTML XHTML
Element names Case-insensitive Must be lowercase
Attribute names Case-insensitive Must be lowercase
Closing tags Optional for some elements Always required
Empty elements <br> or <br /> Must be <br />
Attribute values Quotes optional Must be quoted
Attribute minimization Allowed (checked) Not allowed (checked="checked")

Detailed Syntax Differences

Element Names Must Be Lowercase

HTML (Both Valid)

<P>This is valid</P>
<p>This is also valid</p>

XHTML (Only Lowercase)

<!-- Invalid -->
<P>This is INVALID</P>

<!-- Valid -->
<p>This is valid</p>

All Elements Must Be Closed

HTML (Closing Optional for Some)

<p>Paragraph 1
<p>Paragraph 2
<li>Item 1
<li>Item 2

XHTML (All Must Close)

<p>Paragraph 1</p>
<p>Paragraph 2</p>
<li>Item 1</li>
<li>Item 2</li>

Empty Elements Must Be Self-Closing

HTML

<br>
<hr>
<img src="image.jpg" alt="Description">
<input type="text" name="username">
<meta charset="UTF-8">

XHTML

<br />
<hr />
<img src="image.jpg" alt="Description" />
<input type="text" name="username" />
<meta charset="UTF-8" />

Attribute Values Must Be Quoted

HTML (Quotes Optional)

<div class=container>
<input type=text>
<table width=500>

XHTML (Quotes Required)

<div class="container">
<input type="text" />
<table width="500">

No Attribute Minimization

HTML (Minimization Allowed)

<input type="checkbox" checked>
<input type="text" readonly>
<option selected>Option 1</option>
<script defer src="script.js"></script>

XHTML (Full Attribute Required)

<input type="checkbox" checked="checked" />
<input type="text" readonly="readonly" />
<option selected="selected">Option 1</option>
<script defer="defer" src="script.js"></script>

Element Nesting Rules

XHTML requires proper nesting of elements. Elements must be closed in the reverse order they were opened.

HTML (Tolerates Poor Nesting)

<!-- Not ideal but often works -->
<p><strong>Bold text</p></strong>

XHTML (Strict Nesting Required)

<!-- Invalid -->
<p><strong>Bold text</p></strong>

<!-- Valid -->
<p><strong>Bold text</strong></p>

Special Characters and Entities

XHTML requires proper handling of special characters in content and scripts.

HTML

<script>
if (x < 10 && y > 5) {
    // Code here
}
</script>

XHTML (Use CDATA or Entities)

<script>
//<![CDATA[
if (x < 10 && y > 5) {
    // Code here
}
//]]>
</script>

<!-- OR use entities -->
<script>
if (x &lt; 10 &amp;&amp; y &gt; 5) {
    // Code here
}
</script>

XHTML Document Types

XHTML has three document types (DTDs):

1. XHTML Strict

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

No deprecated elements or attributes allowed. Clean markup only.

2. XHTML Transitional

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

Allows deprecated elements and attributes for transitioning from HTML.

3. XHTML Frameset

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

For documents using frames (deprecated in HTML5).

MIME Types

XHTML documents should be served with the correct MIME type:

Format MIME Type
HTML text/html
XHTML (strict) application/xhtml+xml
XHTML (compatible) text/html

⚠️ Important

When served as application/xhtml+xml, browsers parse XHTML strictly. Any markup error will cause the page to fail completely. Most XHTML documents are served as text/html for compatibility.

HTML vs XHTML: Pros and Cons

XHTML Advantages

  • Well-formed markup: Strict rules ensure clean, consistent code
  • XML compatibility: Can be processed with XML tools
  • Better validation: Errors are caught immediately
  • Forward compatibility: Easier to process programmatically
  • Stricter standards: Enforces good coding practices

XHTML Disadvantages

  • Less forgiving: Small errors break the entire page
  • More verbose: Requires more typing (closing tags, quotes, etc.)
  • Browser compatibility: Strict MIME type not well supported
  • Limited adoption: HTML5 became the standard instead
  • JavaScript issues: Some JS features don't work well with strict XML parsing

HTML5 Advantages

  • More forgiving: Browsers can handle minor errors
  • Modern features: New semantic elements, APIs, and multimedia support
  • Better compatibility: Works consistently across browsers
  • Simpler syntax: Less verbose than XHTML
  • Active development: Continuously updated standard

Converting HTML to XHTML

If you need to convert HTML to XHTML, follow these steps:

Conversion Checklist

  1. Add XHTML DOCTYPE declaration
  2. Add xmlns attribute to html element
  3. Convert all element names to lowercase
  4. Convert all attribute names to lowercase
  5. Quote all attribute values
  6. Close all elements (including p, li, etc.)
  7. Close empty elements with /> (br, hr, img, input, meta, link)
  8. Replace minimized attributes (checked → checked="checked")
  9. Fix nesting errors
  10. Wrap scripts and styles in CDATA sections if needed
  11. Use entities for special characters
  12. Validate with W3C XHTML validator

Before (HTML)

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
<HEAD>
    <TITLE>My Page</TITLE>
    <meta charset="UTF-8">
</HEAD>
<BODY>
    <H1>Welcome</H1>
    <P>This is a paragraph
    <IMG SRC=image.jpg ALT="Image">
    <BR>
    <INPUT TYPE=checkbox CHECKED>
</BODY>
</HTML>

After (XHTML)

<!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" xml:lang="en" lang="en">
<head>
    <title>My Page</title>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
</head>
<body>
    <h1>Welcome</h1>
    <p>This is a paragraph</p>
    <img src="image.jpg" alt="Image" />
    <br />
    <input type="checkbox" checked="checked" />
</body>
</html>

Modern Best Practices

💡 Recommendation

Use HTML5 for new projects. HTML5 is the modern standard and is actively maintained. XHTML knowledge is useful for understanding legacy code and maintaining existing XHTML sites, but new development should use HTML5.

Write HTML5 with XHTML-Style Discipline

You can write HTML5 using XHTML-style best practices without the strict requirements:

HTML5 with Clean Syntax

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>My Page</title>
</head>
<body>
    <header>
        <h1>Welcome</h1>
        <nav>
            <ul>
                <li><a href="/">Home</a></li>
                <li><a href="/about">About</a></li>
            </ul>
        </nav>
    </header>

    <main>
        <article>
            <h2>Article Title</h2>
            <p>This is a paragraph with <strong>bold text</strong>.</p>
            <img src="image.jpg" alt="Description">
        </article>
    </main>

    <footer>
        <p>&copy; 2024 My Website</p>
    </footer>
</body>
</html>

Good HTML5 Practices (Borrowed from XHTML)

  • Use lowercase for element and attribute names
  • Always close elements (even optional ones like p and li)
  • Quote all attribute values
  • Use proper nesting
  • Use semantic HTML5 elements
  • Validate your markup regularly
  • Use consistent indentation

Validation Tools

  • W3C Markup Validator: validator.w3.org
  • HTML5 Validator: html5.validator.nu
  • Browser DevTools: Built-in validation in browser developer tools
  • IDE Extensions: HTML validation plugins for VS Code, Sublime, etc.

Test Your Knowledge