*shrugs* Mine works, and the only things obviously different between your snippet and what I'm using for my cover are that my svg tag also has the following:
and I have a pile of CSS, no standalone="no"
in the <?xml ...>
tag, and the namespace for SVG declared early on:
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:svg="http://www.w3.org/2000/svg">
I'd be surprised if any of those differences mattered, though. It might be a difference in what you're doing in the OPF file.