UPDATE: according to CSS expert and guru, Alan Gresley, the CSS 2.1 spec does not cover precisely how such testcase should be rendered. So, I am removing this testcase and the bug report. CSS 2.1, section 10.3.3 Block-level, non-replaced elements in normal flow states:
"If all of the above have a computed value other than 'auto', the values are said to be 'over-constrained' and one of the used values will have to be different from its computed value."

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.