Instructions: scroll down the thick black bordered box.

Expected results: There should be a 30px (and only 30px in height) aqua gap, space under, below the "Last sentence" of the black bordered box.

Reference:
"If [a block-level non-replaced element in normal flow when 'overflow' does not compute to 'visible'] has block-level children, the height is the distance between the top border-edge of the topmost block-level child box that doesn't have margins collapsed through it and the bottom border-edge of the bottommost block-level child box that doesn't have margins collapsed through it. However, if the element has a non-zero top padding and/or top border, or is the root element, then the content starts at the top margin edge of the topmost child. (...) Similarly, if the bottom margin of the block does not collapse with the bottom margin of its last in-flow child, then the content ends at the bottom margin edge of the bottommost child."
coming from CSS 2.1, section 10.6.3 Block-level non-replaced elements in normal flow when 'overflow' computes to 'visible'