Does HTML5 need an html tag?

No it is not REQUIRED but yes, we should still use it, allow me to explain.

Modern browsers will add this tag it on render if you do not include it BUT it is suggested that you include it anyway. You will find that you may need to use these tags in the future for example, if you wanted to utilize the manifest attribute (for offline browsing) or you wanted to target your whole page with a CSS class in the event that IE gives you issues. The html tag is also where we specify what language our website is using.