I’d bet you that 98+% of users don’t even know you can change the browser font size. For that overwhelming majority of users the size reported by their preferences and their actual preferences do not necessarily align. This is not a defence of setting smaller-than-16px font sizes, but it is a defence of setting larger sizes if testing shows it improves user experience.
The users that need that to be able to read (e.g. vision problems) definitely do know that. It's about accessibility
See: HN, that sets a "10pt" font size, otherwise known as 13px.
At least they have a reason and it fits for me. :)
body {
font-size: larger;
}
.comment, .default, .comhead {
font-size: 14px;
}
.subtext, .pagetop {
font-size: 13px;
}
.title {
font-size: 16px;
}