<width> contains a measurement measured along the
axis parallel to the bottom of the written surface, i.e.
perpendicular to the spine of a book or codex. [10.3.4 Dimensions]
If used to specify the depth 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
‘depth’ axis.