Originally HTML(Hyper Text Markup Language) was designed for sharing information on the internet. At first, the content of the page was text base. As the HTML technology has getting improved, various types of contents such as image, video and audio can be displayed on HTML pages. Draft of standard of latest version, which is HTML5, was defined at Oct 28th 2014 by W3C, and recommendation was announced at 14th Dec 2017 as it's getting improved. The update is still going on.
As it's been updating and supporting various types of media, developers are getting interested in HTML5 games. Especially because HTML5 is not limited to certain devices or platform. It means users can play games from any devices without install. And it can reduce development time for multi platforms.
Before HTML5 arised, there was Flash games. However, Flash games requires installing its plugin or ActiveX and had some security issues. Since Apple announced that they are going to remove Flash from their browser, Flash was getting used less and less and finally Adobe, which is a company that developed Flash, announced that they are not going to support Flash any longer. So HTML5 became the alternative of Flash games.
As web technology is getting advanced, almost anything is possible on the web! We would like people enjoy the technologies with games that is easy to play.