HTML began life as a clever hack of a pre-existing approach. As Tim Berners-Lee explains in his book, “Weaving the Web”:
Since I knew it would be difficult to encourage the whole world to use a new global information system, I wanted to bring on board every group I could. There was a family of markup languages, the standard generalised markup language (SGML), already preferred by some of the world's top documentation community and at the time considered the only potential document standard among the hypertext community. I developed HTML to look like a member of that family.
On The H Open.