What are the differences between HTML5, HTML4.01, and XHTML 1.0?

HTML5 extends the capabilities of HTML4.01 with semantic tags (<nav>, <article>, <aside>), new input types for forms (email, date, range), support for multimedia without plugins (<video>, <audio>), APIs for offline work (Service Worker, Cache Storage), and local storage (localStorage, sessionStorage), as well as APIs for web sockets. The HTML5 syntax is more flexible, allowing omission of certain closing tags and attributes in specific cases.

XHTML 1.0 is a reformulation of HTML 4.01 as an XML application. The main difference is a stricter syntax: all tags must be closed (including self-closing ones like <br />), all attributes must be in quotes, and tag and attribute names are case-sensitive. The goal was to create a more structured and valid syntax for better compatibility with XML parsers. However, in practice, most browsers treated XHTML as HTML, ignoring strict rules if the document was not served with the MIME type application/xhtml+xml.

Comparison table:

Feature	HTML4.01	XHTML 1.0	HTML5
Syntax	More flexible	Strict (XML-based)	More flexible than XHTML but structured
Tag closing	Not always required	Always required (<br />, <img>)	Not always required (<p>, <li>, etc.)
Case sensitivity	Insensitive	Sensitive	Insensitive
Attribute quotes	Not always required	Always required	Not always required in certain cases
Media support	Requires plugins	Requires plugins	Native (<video>, <audio>)
Semantic tags	Fewer (<div>, <span>)	Similar to HTML4.01	More (<nav>, <article>, <aside>)
New APIs	Limited	Limited	WebSocket, Local Storage, Canvas, SVG
Offline support	Almost absent	Almost absent	Cache API, Service Worker
MIME type	text/html	application/xhtml+xml (strict), text/html (as HTML)	text/html
Extensibility	Limited	XML-based	Through APIs and custom elements (future)

Example syntax:

<!-- HTML 4.01 -->
<p>Paragraph.
<img src="image.jpg" alt=Image>

<!-- XHTML 1.0 -->
<p>Paragraph.</p>
<img src="image.jpg" alt="Image" />

<!-- HTML5 -->
<p>Paragraph.
<img src="image.jpg" alt="Image">

HTML5 has become the de facto standard for web development, combining the flexibility of HTML4.01 with new features and partially inheriting the strictness ideas from XHTML, while focusing on compatibility.