Tim Berners-Lee created HTML as a way to view the same data document on many different types of computer from anywhere in the world. His original idea was to facilitate researchers sharing scientific data, though he quickly realised that it could have uses for many types of document.. You can hear Berners-Lee describe his ideas and the developent of the web in The Web Weavers, an video, and in his book, Weaving the Web [Berners-Lee1999].
The primary features of HTML that make it so original an idea are:
format for flexible markup and display
designed to be multi-platform
designed for sharing information as easily as possible
Mosaic browser developed to display pages
Netscape formed by members of Mosaic team
First commercial browser
High quality, yet free for educational use