Document: xmlEncoding property

Returns the encoding as determined by the XML declaration. Should be 
null if unspecified or unknown.
WARNING:
Do not use this attribute; it has been removed from the DOM Level 4 specification and is no longer supported in Firefox 10.0.
Consider the following XML Declaration:
<?xml version="1.0" encoding="UTF-16"?>
Then, the result should be "UTF-16".
Specifications
Browser compatibility