The Document.characterSet read-only property returns the character encoding of the document that it's currently rendered with.
Note: A "character set" and a "character encoding" are related, but different. Despite the name of this property, it returns the encoding.
Users can override the developer-specified encoding inside the Content-Type header or inline like <meta charset="utf-8">, such as with Firefox's View → Text Encoding menu. This override is provided to fix incorrect developer-specified encodings that result in garbled text.
Note: The properties document.charset and document.inputEncoding are legacy aliases for document.characterSet. Do not use them any more.