HTML documents are written in plain text. They can be written in any text editor that allows content to be saved as plain text (although most HTML authors prefer to use a specialized editor that highlights syntax and shows the DOM). Tag names may be written in either upper or lower case.
In the late 1980s, Tim Berners-Lee was working as a physicist at CERN (the European Organization for Nuclear Research). He devised a way for scientists to share documents over the internet. Prior to his invention, communication via the internet was limited to plain text, using technologies such as email, FTP (File Transfer Protocol), and Usenet-based discussion boards.