[css3-page] Negative Page Dimension

Hi,

Although it is not clearly stated at
   - http://dev.w3.org/csswg/css3-page/#page-size
I believe negative values are illegal for page size lengths because they
are illegal for width and height
   - http://www.w3.org/TR/CSS2/visudet.html#the-width-property
   - http://www.w3.org/TR/CSS2/visudet.html#the-height-property

Stating it clearly in the paged media specification may help readers.

Yuzo