XSLT/CSS border style specifications

[Xmplar <info@xmplar.biz>]

How does one control the specifications for the various border styles for tables, etc. I know how to select the line styles - solid, dashed, double, groove, etc - but can the specific metrics for each of these be defined? I want to modify the line widths and spacing between lines for the 'double' style. (I can specify the overall width of line, but is there any finer metrics?) I can't find anything in the XSLT specification, apart from the fact that CSS properties apply. nor is there anything in the DB stylesheets. I'd like to know if this is possible for FO and HTML output.
Thanks,

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Dave Gardiner