What is XHTML?

XHTML (Extensible HyperText Markup Language) is a markup language based on XML, which is a reformulation of HTML 4.01 as an XML 1.0 application.

Main differences between XHTML and HTML:

• XML syntax: XHTML requires adherence to strict XML rules, such as:
  • All elements must be properly nested.
  • All elements must be closed (even empty elements, e.g., <br />).
  • Tags and attributes are case-sensitive (usually lowercase).
  • Attribute values must always be quoted.
• Standards compliance: XHTML is more strict about syntax and requires valid code, which simplifies processing by parsers.
• Extensibility: Due to its basis on XML, XHTML is potentially more extensible.

Practically, XHTML has been largely replaced in web development by HTML5, which combines features of HTML and partially XHTML (e.g., syntax flexibility while maintaining the possibility of strict coding). Nevertheless, understanding XHTML principles is useful for understanding the evolution of web standards and working with XML parsers.