In future, in the same like today, if any user still wants to use Flash they will have to enable Flash manually.
HTML5 and Flex is the future.
HTML5 is a cooperation between the World Wide Web Consortium (W3C) and the Web Hypertext Application Technology Working Group (WHATWG).
The latest versions of Apple Safari, Google Chrome, Mozilla Firefox, and Opera all support many HTML5 features and Internet Explorer 11.0 will also support HTML5 functionality.
The mobile web browsers that come pre-installed on iPhones, iPads, and Android phones all have excellent support for HTML5.
Flexbox (flexible box) is a layout mode of CSS3. Using this mode, you can easily create layouts for complex applications and web pages. Flexbox layout gives complete control over the direction, alignment, order, size of the boxes.
Following are the browsers that support Flexbox.