Comment by enriquto

Because it's unnecessary in html5, just like closing li, td, tr tags. When you use implicit tags (body, header, etc) and auto-closing tags (li, p, tr, td,...) then html becomes refreshingly lean and human-editable.

EDIT: for a nice summary, look at google's html style guide. Section 3.1.7 is particularly neat [0].

[0] https://google.github.io/styleguide/htmlcssguide.html#Option...