HTML5 (No space between "HTML" and "5")is a markup language used for structuring and presenting content on the World Wide Web. It is the fifth and current version of the HTML standard.
The CSS was started to develop around 1998. And till 2009, it was under development. The first working draft of CSS3 came in 19-01-2001. but it was initially declared early in the June 1999.
Bootstrap 4 alpha 1 was released on 19th August 2015. Since then there have been many alpha releases. And finally after that on 10th August 2017, Bootstrap released the first beta for the version 4 of Bootstrap.