I am writing a book in epub and the problem is that I want the text to be in the middle of the page. I tried style="top:50%", it works in Sigil, but not in the sony my reader app. I heard that sony uses a special type of epub, so if you know something about it please tell me.
I have no knowledge of Sony specifics, but in general you can't expect this kind of positioning logic to work well across all ebook readers. Anything involving the CSS 'position" property with a value other than static (the default) is likely to be problematic, especially with EPUB2 readers. I'm guessing this would work fine in iBooks, or Readium, did you try that?
The safest alternative is an image.
If you wait 3-18 months, most ebook readers will have been upgraded and you can expect the great majority of them to support what you are trying to do.