In the UK, there's a group of tax-efficient savings accounts known as ISAs (Individual Savings Account). They can be used to save a certain amount (new) money each year, free of tax on interest (cash ISAs), and free of dividend and capital gains taxes (share ISAs). They are generally recommended and widely used, and their use is actively encouraged by the UK Government. According to Wikipedia, they're broadly similar to Roth IRAs and Roth 401(k) accounts in the USA.
How would a US citizen who's a UK permanent resident treat an ISA on their US tax return? For example, do they get any of the tax benefits against their US return, or just the UK one? And do stocks-and-shares ISAs get any friendly treatment, or do they run the risk of being classed a PFIC (Passive Foreign Investment Company) + risk needing a form 8621 filling out?