In order to define charset for HTML 5 doctype, which notation should I use?
Short:
meta charset="utf-8"
Long:
meta http-equiv="Content-Type" content="text/html; charset=utf-8"
See the section Specifying the document’s character encoding in the HTML 5.1 spec:
[…] using a
meta
element with acharset
attribute or ameta
element with anhttp-equiv
attribute in the encoding declaration state.
So both ways are fine. (But don’t use both in the same document.)