The caffeine content of coffee varies greatly, depending on the beans, how they're roasted, and other factors, but the average for an 8-ounce cup is about 100 milligrams (mg).
No, 'cat' is a common noun, a word for any cat of any kind. A proper noun is the name of a person, a place, a thing, or a title; for example: Cat Deeley, host of TV series 'So You Think You Can Dance'.
I was always taught that proper nouns were the names of particular (i.e. specific one of kind items) persons, places, or things, and that all proper nouns should be capitalized. The names of the seasons certainly meet this criteria and I, for one, will, as I always have throughtout my 60 yr life, capitlaize them.