Vertical Margin Collapse

Declaring a bottom margin on the top div of 30 pixels, and a top marging on the bottom div of 20 pixels should result in a total vertical space of 50 pixels between the div.

It doesn't. CSS collapses them to the larger setting, in this case 30 pixels.

Hello I'm div one. As long as my height is a multiple of 21, i.e 42, 63, 84 etc. I should follow the vertical rhythm.
Except this div is 84px high and appears to be one pixel to short?