Obsolete since Gecko 7.0 (Firefox 7.0 / Thunderbird 7.0 / SeaMonkey 2.4)
This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.
Prior to Gecko 7.0 this method showed up as present, due to bug bug 9850, it always only returned null. The only workaround is to create a text node, CDATA section, attribute node value, etc. which has the value referred to by the entity, using Unicode escape sequences or fromCharCode() as necessary.