Responsive Foundation 6.0.0

The minimum font size in pixels for headings to use on mobile. Unless you're a designer and you know what you're doing, leave this at $font-size-base. Anything lower means you'll need to compensate with other design elements to preserve the visual hierarchy - think uppercasing, color, stuff like that. This directly affects the <h6> tag. It indirectly affects all other heading font sizes as well by serving as the lower limit to our scale.