Which version of Extensible Hypertext Markup Language (XHTML) is not backward compatible with HTML

Which version of Extensible Hypertext Markup Language (XHTML) is not backward compatible with HTML?

The correct answer and explanation is:

Correct Answer: XHTML 2.0

Explanation (300 words):

XHTML (Extensible Hypertext Markup Language) is a reformulation of HTML using the stricter syntax rules of XML (Extensible Markup Language). Various versions of XHTML have been developed to improve structure, flexibility, and interoperability in web documents. The major versions of XHTML include XHTML 1.0, XHTML 1.1, and XHTML 2.0.

XHTML 1.0 and XHTML 1.1 are largely backward compatible with HTML 4.01. That means web browsers that support HTML could still interpret and display XHTML 1.0/1.1 documents correctly—especially when the document is served with the MIME type text/html.

However, XHTML 2.0 was a complete departure from the HTML line of development. Unlike its predecessors, XHTML 2.0 was not backward compatible with HTML. It introduced several major changes that broke compatibility, such as:

• Removal of many legacy HTML elements (e.g., <img>, <form>, and <a> were either redefined or replaced).
• Different methods for linking and navigation (the <a> tag with href was replaced by other mechanisms).
• New forms module that worked differently from the traditional HTML form model.
• No support for browser scripting events like onclick, making it less friendly to client-side JavaScript.

Because of these fundamental differences, existing HTML or XHTML 1.x documents would not work correctly if interpreted as XHTML 2.0.

Due to its strictness and incompatibility, XHTML 2.0 faced significant criticism. It was seen as impractical for the web development community, which valued backward compatibility. Consequently, in 2009, the W3C officially discontinued development of XHTML 2.0 and shifted focus to HTML5, which aimed to blend the flexibility of HTML with some XML-like strictness, but with strong support for existing content and practices.

In summary, XHTML 2.0 stands out as the version of XHTML that broke away from HTML compatibility, making it unsuitable for backward compatibility and broad adoption.