This is implemented in 4.1.x. There are separate style-properties for each of the heading levels 1 through 6, and except for XenForo 2.x heading levels 3-6 inherit empty values from heading level 2 (to emulate the current behavior). The font-size of each level is now taken from the appropriate style property.
Since XenForo 2 does not differentiate between undefined border and border: none in the style properties, XenForo 2 headline properties do not inherit empty values from heading level 2. However, in XenForo 2, the default value for all heading font-sizes is a calculation based on the value of @fontSizeNormal. Admins may alter the calculation parameters or choose specific values when customizing font-size.