HTML 5 is a markup language used for structuring and presenting content for the World Wide Web. It was introduced by Tim Berners-Lee in 1980 at CERN as a way to share documents. By 1989, Berners-Lee proposed a hypertext system using the Internet. In late 1990, he specified HTML and wrote the first browser and server software. HTML forms the foundation of web development and is used in client-side programs like JavaScript and server-side languages such as PHP.