XHTML (Extensible HyperText Markup Language) is a language for structuring and formatting content on the web that was developed as a further development of HTML (Hypertext Markup Language). XHTML is an XML-based language and follows the rules of XML syntax, which makes it a stricter and more consistent language than HTML.

XHTML was developed to address the shortcomings of HTML and improve compatibility with other technologies such as XML and CSS. XHTML enables a clearer separation of content and layout, which can help to make websites more accessible and barrier-free. XHTML also provides stricter rules for the design of websites, which helps to ensure that the source code is better structured and easier to maintain.

In contrast to HTML, XHTML uses a stricter syntax and requires that all tags are correctly nested and closed. Errors in the XHTML code can lead to a website no longer being displayed correctly or not being rendered correctly.

Although XHTML was widely used in the past, it is now often replaced by HTML5. HTML5 offers similar functions and advantages to XHTML, but is more flexible and also supports multimedia and interaction functions.