In schema v1.2 in <define name="draw-enhanced-geometry-attlist"> the attribute <attribute name="svg:viewBox"> is optional. But the element draw:enhanced-geometry has a lot of attributes and child elements, which use coordinates or length without unit. To determine their values with unit, the values of svg:viewBox are mapped to the position and size of the shape.
In case the attribute svg:viewBox is missing, current implementations act different. This was discussed in thread https://www.oasis-open.org/apps/org/workgroup/office/email/archives/201303/msg00036.html. The thread has an example document attached.
There are several solutions imaginable:
(1) Define a default viewBox, for example svg.viewBox="0 0 21600 21600". This solution is used in AOO.
(2) Define, that the viewBox is calculated from maximal and minimal values actually used. Calligra seems to do something of this kind
(3) Add, that the attribute svg:viewBox may not be missing, if there exists any position or length value without unit in the attributes and child elements.
(4) Add, that for interoperability the attribute svg:viewBox should not be missing, and missing attribute svg:viewBox gives implementation dependent results.