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Backgrounds & Borders allows for percentages in border-width and they refer to
the width of the containing block:
However, as the testcase demonstrates, they are not yet supported in Webkit.
As at the time of this comment, that draft is labeled 'W3C Candidate Recommendation 15 February 2011'.
Whereas in the 'Editor's Draft 17 May 2011':
... the prose about percentages for border widths has been removed.
See this thread on www-style for some context:
Closing as the CSS editor indicates this is not a planned feature.
Please reopen if that changes.