S3,
For some federal government agencies, they use the Chicago style manual as the
basis of writing government reports. That manual should cover most of the
questions relating to word usage and punctuation rules.
But I doubt if it covers all of the current slang words like "flossing",
John, as Seraphita points out, "they" as a singular pronoun has been in popular
use for a long time, including by some top-notch writers. It was declared a
solecism in the 18th century by overly persnickety grammarians, but that didn't
succeed in stamping it out; and it's currently undergoing a
