For a non-US person (or offshore hedge fund for example) who has never been US and has no any relationship with US, if he open a stock brockage account with any brockage firm in US (such Interactive Brokers, etc), my understating is that the interest income is US tax free but dividends is not US tax free. How about stock capital gain? Is it US tax free as well? Of it is subject to US capital gain tax? Does any one have any experience/idea?