Anything that's faultless doesn't have a single flaw — it's perfect. It's surprising to learn that an actor is Welsh when she can speak with a faultless American accent.
Faultless things have no faults or imperfections, like a comedian's faultless imitation of the president, which sounds identical to the real person. You can also use this adjective to describe someone who's not at fault, or can't be blamed: "I know someone ate the brownies, but I wasn't home and I'm utterly faultless." The fault part of faultless has a Latin root meaning "deceive or disappoint."