Tagging with external database identifiers

How should we indicate that a tag represents an entry in an external database?

Please see:

http://wiki.openstreetmap.org/wiki/Template_talk:KeyDescription#Website_and_URL_pattern

and comment at:

http://wiki.openstreetmap.org/wiki/Template_talk:KeyDescription#Website_.26_url_pattern.2C_redux

Why not like the wikipedia entry in your example’s infobox (http://www.openstreetmap.org/node/355916994)?