what will be the right answer for HTML5 support in browsers ?
That question is much broader than one may think, since there are many different areas involved. HTML5 itself is the last major release and the different parts were developed separately from then (known as the “HTML Living Standard”). Mostly it’s more useful to look at the browser support of specific features with https://caniuse.com/ or the browser compatibility tables at the end of every MDN article.
That being said, if I had to put a specific number on the browser support of HTML5 I would say: “Internet Explorer 9 and later”. It supports most of the new tags like <nav> or <article> and the media tags <audio> and <video>.
I hope that helps,
Michael