The HTMLFormElement.acceptCharset property represents the character encoding for the given <form> element.
The specification allows a single case-insensitive value of "UTF-8", reflecting the ubiquity of this encoding (historically multiple character encodings could be specified as a comma-separated or space-separated list).
This reflects the value of the form's accept-charset HTML attribute.
A string which may be a case-insensitive match for UTF-8.
let charSet = document.forms["my-form"].acceptCharset;