There is a decent case for controlling text size using cookies. This is because people often want different text sizes on their different devices. This is difficult to deal with via settings on the account. Fastmail was probably also motivated to handle it that way because some other settings (such as persistent login) were already managed using cookies. Perhaps, there could be an option "fixed text size on all devices" or something to provide a different solution for those who do not want Fastmail to use persistent cookies.
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