Translation:My husband is an artist, and his life is art.
"Art is his life" says exactly what the sentence implies in Russian - his entire life is consumed/occupied with art.
But saying "his life is art" implies that you're looking at it from an outside perspective, i.e. a documentary, commenting Warhol style that "everything in this guy's life is art" whether or not the guy knows it.
Right, so for и его жизнь -искусство it won't accept "and art is his life" and wants me to put "and his life is art". This leads me to think that either: DL is wrong and should accept both, or: I'm not understanding the Russian correctly and it means something like "his life itself is, literally, art". My assumption was that the Russian meant to the effect that "and art plays an extremely important part/central role in his life" - in which case my (rejected) translation is more natural (and unambiguous) in English.
Something like "Ван Гог - известный голландский художник." It's certainly appropriate to translate "художник" as "painter" and it should be accepted. But English "painter" can be ambiguous because unlike "художник" it doesn't necessarily mean a painter of art (or something that was supposed to be art, since you mentioned Van Gogh...), so it makes sense to have "artist" as the primary translation.
Context is everything. Artist by itself usually means a painter. Painter by itself usually means an artist. But both words are ambiguous in English and both should be equally acceptable.
Sometimes just an adjective makes it clear. If you say someone is a "famous painter," no one would think you meant a house painter. But a context can make an "artist" a musician (as in iTunes list menus) or a writer, dancer, cabinet maker, cook or basket weaver.
Oinophilos, you yourself made the same point I did a while back. Seems we both instinctively felt that "and art is his life" was the natural English translation. I dare say plenty of others are getting the answer 'wrong' plus, as I said, the accepted translation unintentionally instils an element of ambiguity to the English. Pedantic of me to point that out, possibly, but unfair I think to accuse me (us?!) of pettiness in pointing out a more natural world order, since their are likely non-native English speakers doing this course too. Cheers.