The WHATWG HTML spec requires UTF-8 for conforming documents and scripts [WHATWG 4.2.5.4]. In both HTML specs, charset declarations, if provided, must be UTF-8 [4.2.5].
If the transport, content-type, lack of charset declaration, and sniffing fail to determine an encoding, both specs use defaults based on the configured locale, for English that's windows-1252 [WHATWG: 12.2.3.2 W2C: 8.2.2.2]. latin1/ISO-8859-1 is prohibited. [WHATWG: 12.2.3.3 W3C: 8.2.2.3].
If the transport, content-type, lack of charset declaration, and sniffing fail to determine an encoding, both specs use defaults based on the configured locale, for English that's windows-1252 [WHATWG: 12.2.3.2 W2C: 8.2.2.2]. latin1/ISO-8859-1 is prohibited. [WHATWG: 12.2.3.3 W3C: 8.2.2.3].