[CSS 3] max()

While generally we want to set font-sizes in rem, there's a particular issue on mobile iOS, 

where if the field text is not at least 16 pixels, it will cause a zoom into the field,

which may not be desirable as it can remove the label out of the viewport.

font-size: 16px; // fallback
font-size: max(16px, 1em);

so, if `1em` is no larger than 1px, then use 16px.