It seems that Opera and Prince handle percentage height values in different way. If you specify height of html element

html {height:50%}

Opera produces box that has height equal to half of page height.
Prince and Mozilla seem to ignore percentage values in this case.
The question is which one does it right?
According to CSS2.1 specs height should be equal to half of viewport's height but viewport is defined for continuous media only, not paged.
So I assume Prince is right. In any case I am not completely sure.
Here is testcase
http://geocities.com/csssite/operabugs/bug55.xhtml

Hmm, it seems that the containing block in which the root element lives is the page area for paged media, implying that a height of 50% on the root element should indeed give it a height that is 50% of the page height.

I think that Opera has this one right

For the record, this bug has been fixed in Prince 5.0.

Thank you for reporting the issue