The markup language of web pages, created by Tim Berners-Lee around 1990. Net artists of the 1990s used it as a medium in its own right.
And thankfully, with HTML, you could view source. So I could look at what other people were doing, understand, oh, this is how you take two images and have them swap when you roll over.
Joshua Davis, 1990s Net Art: Auriea Harvey, Joshua Davis, LIA, Draves
I think artists like JODI were doing really interesting work in the browser that I consider to be generative, using only standard HTML, form elements and things like that.
Casey Reas, Casey Reas on the History of Generative Art - Part 2