Although she has dug up information about things like the history of the colon, Truss is so uninterested in the actual rules of punctuation that she even names the ones she flouts–for example, the rule that semicolons cannot be used to set off dependent clauses. (Unless you are using it to disambiguate items in a list, a semicolon should be used only between independent clauses–that is, clauses that can stand as complete sentences on their own.) That is the rule, she explains, but she violates it frequently. She thinks this makes her sound like Virginia Woolf. And she admits that her editors are continually removing the commas that she tends to place before conjunctions.
Why would a person who is not just vague about the rules but disinclined to follow them bother to produce a guide to punctuation? –Louis Menand –Bad Comma (The New Yorker)