In order to define charset for HTML5 Doctype, which notation should I use?
Short:
was introduced with/for HTML5.
As mentioned in the documentation, both are valid. However, is only for HTML5 (and easier to type/remember).
In due time, the old style is bound to become deprecated in the near future. I'd stick to the new .
There's only one way, but up. In tech's case, that's phasing out the old (really, REALLY fast)
Documentation: HTML meta charset Attribute—W3Schools