After googling around a bit, that content-style-type meta tag inside an html document is not used or needed in epubs and most things just ignore it. Unfortunately, epubcheck is not one of those things that ignore it.
You can safely remove that line from untitled.html before conversion and all should work just fine. I will update epub3-itizer to auto-remove meta tags like that for the next release. For now just remove it in Sigil.
So thanks for your bug report!
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