Goodbye Flash and good riddance. HTML5 is so much more than fancy animations and sliding navigation menus; it will basically work on any device used to access the web.
Building a website or custom web app in HTML5 allows you to create extremely interactive user experiences that take accessibility to a whole 'nother level. It also provides native support for video and audio allowing you to say adios to the nighmares of getting your media to play correctly.
Websites are much more accessible on smartphones thanks to the implementation of HTML5 by nearly all mobile web browsers. The support for HTML5 on smartphones also makes it easier to create native app-like interfaces—hence the popularity of responsive web design.