Is it necessary to use html 5 to implement with Java EE 7 for a chat application

I came to know about websocket in Java EE 7. so i was looking how can i implement a chat application in Java EE 7 and I found this site

Here they are mentioning that HTML 5 is required. Can anybody tell me is it necessary to use HTML 5 . if it is then why.. can i use HTML 4 ??