Either:
One or more 'glyphRef' elements.
Any number of descriptive elements and at most one 'mpath' element, in any order.
Any number of descriptive elements
and exactly one light source element, in any order.
Any number of descriptive elements
and exactly one light source element, in any order.
Any number of descriptive elements and at most one 'font-face-src' element, in any order.