<depth> contains a measurement measured across the
spine of a book or codex, or (for other text-bearing objects) perpendicular to the measurement given by the
‘width’ element. [10.3.4 Dimensions]
If used to specify the width of a non text-bearing
portion of some object, for example a monument, this element
conventionally refers to the axis
facing the observer, and perpendicular to that indicated by the
‘width’ axis.