Answer 1

There does not exist html6 or css4
It's HTML5 and CSS with different levels of features

https://www.w3.org/TR/html52/
HTML 5.2 - World Wide Web Consortium

https://www.w3.org/TR/CSS/
CSS Snapshot 2017 - World Wide Web Consortium

https://www.reddit.com/r/html5/comments/60o8mb/html6_release_date/
HTML6 release date? : html5 - Reddit

https://rachelandrew.co.uk/archives/2016/09/13/why-there-is-no-css4-explaining-css-levels/
Why there is no CSS4 - explaining CSS Levels - Rachel Andrew

https://www.reddit.com/r/webdev/comments/57a3yp/why_there_is_no_css4_explaining_css_levels/
Why there is no CSS4 - explaining CSS Levels : webdev - Reddit

https://www.sololearn.com/discuss/746150/?ref=app