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The word act, as used in the term Act of Congress, is a common, not a proper noun. The capitalization of the word act is deprecated by some dictionaries and usage authorities. Some writers, in particular the U.S. Code, capitalize Act.
Capital letters are not really an aspect of punctuation, but it is c onvenient to deal with them here. The rules for using them are mostly very simple. (a) The first word of a sentence, or of a fragment, begins with a capital letter: would like to be the president of a big company. In the first, the title the President is capitalized because it is a title referring to a specific person; in the second, there is no capital, because the word president does not refer to anyone in particular.