CSSRule
String encoding
The character encoding specified by the @charset rule. If you set this property to an illegal value, a DOMException with a code of SYNTAX_ERR is thrown. If the rule or style sheet is read-only, an attempt to set this property throws a DOMException with a code of NO_MODIFICATION_ALLOWED_ERR.
This interface represents an @charset rule in a CSS style sheet. Consult a CSS reference for details.