Function encode_with_html_fallback 

fn encode_with_html_fallback<'a>(
    input: &'a str,
    encoding: &'static Encoding,
) -> Cow<'a, [u8]>

Encode a string with the form’s encoding, converted to a byte sequence. https://encoding.spec.whatwg.org/#encode

Characters that can’t be encoded in the given charset are replaced with HTML decimal numeric character references (e.g. 😂 → 😂).