Websites are built using the coding language XHTML (Extensible Hypertext Markup Language). Although it employs a more rigid XML-based structure, it is comparable to HTML. In 2000, XHTML's initial version (1.0) became defined. For a while, the most popular language for building webpages was XHTML. HTML5 has since supplanted it in use.

Why XHTML?

Browsers became more forgiving in how they interpreted website source code as HTML developed over the first few decades of the web. As a consequence, webpages appeared differently in different platforms. The uniform appearance of websites in different platforms was one of the primary objectives of XHTML.

Websites written in XHTML must adhere to a rigorous XML grammar because XHTML is built on XML rather than HTML. A website that adheres to the "XHTML Strict" doctype (DTD) cannot have any mistakes or incorrect elements, giving the web browser no room for interpretation. The "XHTML Transitional" doctype, which does not demand flawless grammar and even permits HTML 4.01 elements, was used by the majority of XHTML websites.

XHTML served as the de facto coding language for web programming from about 2001 to 2011. While the majority of writers used flexible doctype, some used a rigid XHTML DTD. The web ultimately returned to HTML because the majority of web writers favored a more adaptable language. The W3C formally suggested HTML5 in 2014. The majority of current computers continue to handle both HTML and XHTML.