As today most popular platform mobile devices so HTML5 support cross platform.Its working process is design as to work whether you are using a PC, or a Tablet, a Smartphone, or a Smart TV.The current version delivers everything from animation to graphics, music to movies, and can also be used to build complicated web applications.
Answers (1)
Most browsers will still support both, but it's like asking "Why should i upgrade my skype or browser ?" It offers more stability, compatibility and allows us to do things faster, and removing excess things that no one uses anymore and that was pretty much done with css for a long time. As the internet grew, we needed more ways to portray things in html, like different elements, flash etc, and these run much smoother on HTML 5. And there's nothing you can do in HTML that you can't do in HTML 5, but there are several things that you can't do vise versa, in short, some of the syntax was removed and a few other things were added, but it's still pretty much just an upgrade that adapts more to what we really use nowadays .