Which of the following is not a web browser?

A web browser is software used to access and view websites on the Internet. Examples include:
Firefox → A popular open-source web browser.
MacWeb → An early web browser for Macintosh computers.
Netscape → One of the first widely used web browsers in the 1990s.
On the other hand:
XHTML (Extensible Hypertext Markup Language) is not a web browser.
It is a markup language used to structure and format web pages, similar to HTML.
Browsers like Firefox, Chrome, or Netscape interpret XHTML to display web content, but XHTML itself is not software.