I have inquired about the possibilities. As a rule, we do not record subjective classification of objects. But it would be fine if you used British English and lower case letters for the key and the value.
Oh, what I overlooked in the example: the tag "elevation"="1 m s.l.m. circa" is also not a known key and the value is hardly evaluable. Please have a look at the wiki page for the key ele (see https://wiki.openstreetmap.org/wiki/Key:ele).