A string specifying the filepath name of an HTML template file. If non-null and not an empty string, this file will be "played", if it is not already on the "play" list. If NULL or an empty string, the site will still be registered, but can be overridden by a similar site name with a specified filename.
site
A string specifying the site name to register the site function callback on.
value
A string to substitute for the specified data site. The string is copied locally.