Following on from keithjgrant's comment here.
Assuming that tags will be in French (which I believe they should), should they include the definite article? I.e. grammaire or la grammaire?
(I say grammaire, what say you?)
By definition of what is a word, there's no reason a keyword should be two words.
It is not a “keynoun”, and therefore there's no question whether nouns should get an article or not: it is a keyword, and words do not need articles. On the contrary, if you add an article, it's not one word anymore.
Definition: Word: “the smallest free form that may be uttered in isolation with semantic or pragmatic content” (Wikipedia EN)
In the French Wikipedia: “suite de signes […] pouvant être distingués par un séparateur (blanc typographique à l'écrit, pause à l'oral)” - Wikipédia FR. The Académie Française's dictionary is not as specific with the separator, but its definition does not conflict with Wikipedia's.
Ok, I think that's enough formal justification for what everyone agreed on by gut feeling ;)