In http://lists.w3.org/Archives/Public/public-css-testsuite/2012Oct/0009.html, Gérard Talbot gave me some good suggestion, copy here for your reference:

[[
5-
Generally speaking, absolute positioning with 'top: auto' and 'left:
auto' (implicit on #container) is to be avoided because of multiple
renderings allowed (when determining the static position and if margin
collapsing occurs) by CSS:

"
But rather than actually calculating the dimensions of that hypothetical
box, user agents are free to make a guess at its probable position.
"
http://www.w3.org/TR/CSS21/visudet.html#abs-non-replaced-width
and
http://www.w3.org/TR/CSS21/visudet.html#abs-non-replaced-height

On the other hand, top: auto and left: auto for relatively positioned
element is preferred because more reliable, trustworthy.

6-
It is preferable (although optional) to list declarations in
alphabetical order. This helps review.

7-
<style type="text/css">
is not a mistake but isn't needed for HTML5.

Trailing / at closing tag are not necessary for HTML5.

You are encouraged to always use/reuse the templates available at
http://wiki.csswg.org/test/format#template-for-new-tests
]]

I have no good understanding of the absolute position, so just keep it there and work out a revision:

http://zqzhang.github.io/review/box-sizing-001-review.html

http://zqzhang.github.io/review/box-sizing-001-ref-review.html