XHTML or eXtensible HyperText Markup Language is a markup language that has the same expressive possibilities as HTML, but a stricter syntax. Whereas HTML is an application of SGML, a very flexible markup language, XHTML is an application of XML, a more restrictive subset of SGML.

Most web pages on the Gustavus website use XHTML because the Gustavus template is written in valid XHTML 1.0 Transitional.