Most readers probably don't follow the MUST about the requiredExtensions attribute for SVG #1087
Description
SVG Reading System conformance
It MUST recognize the value "http://www.idpf.org/2007/ops" of the requiredExtensions attribute as representing the occurrence of XHTML Content Document fragments (e.g., when the attribute is included on the foreignObject element or children of the switch element).
Presumably there was a good historical reason for this that's lost on me, but I doubt most EPUB readers do anything with this MUST.