Fix meta charset for HTML5

"This <meta> element is a synonym for the pre-HTML5 <meta http-equiv="Content-Type" content="text/html; charset=IANAcharset"> where IANAcharset corresponds of the value of the equivalent charset attribute. This syntax is still allowed, although obsolete and no more recommended."

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/meta#attr-charset