Stands for "Extensible Hypertext Markup Language." XHTML is markup language used to create webpages. It is similar to HTML but uses a more strict XML-based syntax. The first version of XHTML (1.0) was standardized in 2000. For several years, XHTML was the most common language used to create websites. It has since been superseded by HTML5.
As HTML evolved over the first few decades of the web, browsers became increasingly lenient in how they parsed webpage source code. The result was that websites were rendered inconsistently between browsers. One of the main goals of XHTML was to ensure webpages looked the same across multiple browsers.
Since XHTML is based on XML rather than HTML, webpages coded in XHTML must conform to a strict XML syntax. A webpage that uses the "XHTML Strict" doctype (DTD) cannot contain any errors or invalid tags, leaving no ambiguity for the web browser. However, most XHTML sites used the "XHTML Transitional" doctype, which does not require perfect syntax and even allows HTML 4.01 tags.
From 2001 to about 2011, XHTML was the standard markup language for web development. Some developers used a strict XHTML DTD, though most used transitional doctype. Since most web developers preferred a more flexible language, the web eventually transitioned back to HTML. In 2014, HTML5 was officially recommended by the W3C. Most modern browsers still support both HTML and XHTML.