SVG font-face-src required in the schema but optional in the spec

Dear colleagues,

‘font-face-src’ appears to be optional, since the font-face
content model as described in
http://www.w3.org/TR/SVG/fonts.html#FontFaceElement:
is:

"Any number of descriptive elements and at most one ‘font-face-src’
element, in any order."

But the DTD fragment available at http://www.w3.org/TR/SVG/svgdtd.html#DTD.1.39
requires ‘font-face-src’.

<!ENTITY % SVG.font-face.content
    "( ( %SVG.font-face-src.qname;,
         ( %SVG.Description.class; %SVG.font-face.extra.content; )* )
       | ( ( %SVG.Description.class; %SVG.font-face.extra.content; )+,
           %SVG.font-face-src.qname;,
	   ( ( %SVG.Description.class; %SVG.font-face.extra.content; )* )? ) )"
>

This issue was raised against IDPF EPUB3.  See
https://code.google.com/p/epub-revision/issues/detail?id=208

Regards,
Makoto