Backgrounds & Borders allows for percentages in border-width and they refer to the width of the containing block: http://www.w3.org/TR/css3-background/#the-border-width However, as the testcase demonstrates, they are not yet supported in Webkit.
> http://www.w3.org/TR/css3-background/#the-border-width 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': http://dev.w3.org/csswg/css3-background/#the-border-width ... the prose about percentages for border widths has been removed. See this thread on www-style for some context: http://lists.w3.org/Archives/Public/www-style/2011May/0371.html
Closing as the CSS editor indicates this is not a planned feature. Please reopen if that changes.