I think this regressed with bug 69709 which included the border in the width HTML table for both fixed and percent length. The ChangeLog and the provided links mention the need for fixed length only. I could not find anything relevant in the discussions linked to the bug or the spec. Firefox and IE9 don't seem to include the border in the length in this case.
Created attachment 114767 [details] test case: the pink and green borders should not be outgrow the black and blue ones.
Please replace HTML with CSS in my previous analysis (pretty obvious from the change and the test case that we are talking *CSS* table).
Created attachment 114789 [details] Proposed change: avoid adding the border to percent length.
You're absolutely right - percent tables should have been excluded in but 69709.
Comment on attachment 114789 [details] Proposed change: avoid adding the border to percent length. r=me
Comment on attachment 114789 [details] Proposed change: avoid adding the border to percent length. Thanks, David!
Comment on attachment 114789 [details] Proposed change: avoid adding the border to percent length. Clearing flags on attachment: 114789 Committed r100661: <http://trac.webkit.org/changeset/100661>
All reviewed patches have been landed. Closing bug.