It does not look like this plugin will work as advertised for HTML/XML files except in specific cases. There does not appear to be any logic in the code to translate the IANA charset name into a Vim encoding name, e.g. it will try to use "windows-1252" as the fenc instead of translating this to "cp1252".
Additionally, the script has had no updates since the initial upload in 2002. Probably these solutions are better. At least, they are better maintained: