[CSS21] WD 8.5.1: negative borders (presumed editorial)

The description of <length> values for <border-width> says

  # Explicit border widths cannot be negative.

but doesn't say what the behaviour should be if a negative explicit
border width is requested.

If the intention is that the "illegal values" rule of §4.2 applies,
then I suggest rewording to

  | Negative lengths are <a href="syndata.html#illegalvalues">illegal</a>.


pjrm.