Rule of thumb: body text should never be smaller than 16px or (preferably, though the difference is nuanced) 1em as the browser gives it at the root. (This holds for form fields too, with a more concrete technical reason: iOS will zoom in on the field on focus if its font-size is less than 16px.) 12px is pretty much the lower bound of what is ever acceptable, and only in small quantities and for specific purposes, and it’s better not to go down that small.