Q:

Can't define the right margin-top's %

I have a series of containers, all set to height 100% starting from 'body'.  
I set 'article':
article {
  width: 100%;
  height: 50%;
  margin: 0;
  margin-top: 25%;
  padding: 0;
}

The height works fine taking half of its parent's (or screen, doesn't matter) height, but the margin-top is definitely not 25%, more like three times it.  
Live link: http://no-plans.com/temp/wp-tobias/wordpress/?p=51
Disclaimer: still dirty stylesheet, there might be inherited conflicts but I can't find any.  I also tried to resize and put the margin to other parent divs, same issue.  

A:

As stated in the comment, margin percentages are relative to container's width: http://w3.org/TR/CSS21/box.html#margin-properties. 
You can solve by absolute positioning and top (here the percentages are relative to container's height).