[CSS2.1][CSSOM] "Used Value" and "auto"

I'm working with Ryosuke Niwa on correcting the behavior of
getComputedStyle(elem).top/left/right/bottom in Chrome. It led us to a
discussion about whether or not "auto" is a valid used value, and whether
it should be returned by getComputedStyle if get computed style is
returning a "used value" for a specific property. The discussion is
https://bugs.webkit.org/show_bug.cgi?id=29084 and
https://www.w3.org/Bugs/Public/show_bug.cgi?id=16389

Basically, IE9/FF/Opera/Webkit all currently have different behaviors with
regards to this test:
https://bug-29084-attachments.webkit.org/attachment.cgi?id=151525
Opera 11.6: Passes all tests as written
FF13: Converts "auto" to pixels so fails when expecting "auto".
IE9/IE10: incorrectly accounts for padding so fails on the padding tests.
Webkit: doesn't convert to pixels so fails when expecting pixels.

Obviously, we want Webkit to return pixels when percentages are specified.
This makes us believe "used value" should be used, and more specifically,
that the "resolved value" for top/left/bottom/right should be brought into
line with margin/padding/border (in that they all use "used value" if
display is not "none"). However, we're unsure if "auto" is a "used value",
or if "used value" should be pixels, as is seen in FF13's current
implementation.

I summed up the argument thusly:


According to http://www.w3.org/TR/CSS2/cascade.html#used-value, " The used
value is the result of taking the computed value and resolving any
remaining dependencies into an absolute value." and then subsequently "A
used value is in principle the value used for rendering,"
Is "auto" an absolute value or the value used for rendering? I don't
personally believe so.
Is "auto" a useful value to return? I personally don't believe so.
Is "auto" consistent with MOST of the other browsers? yes.

The crux of my question is this: is "auto" considered a "used value"? If
not, should it be converted to pixels for top/left/bottom/right (like
FF13)? Any insights here would be extremely helpful.

-- 
Mike Sherov
Lead Developer
SNAP Interactive, Inc.
Ticker: STVI.OB