This is a proposal for a different interpretation of negative -moz-border-radius values. Basically, negative values would "eat in" to the box, in a way opposite to how positive -moz-border-radius values "round off" the box. See http://www.bath.ac.uk/%7Epy8ieh/m/border-radius-negative.html ...for an illustrated explanation. This is a low priority item, but I'm sure skin authors would fine it useful.
Probably won't make it into the beta, maybee next release.
(Hixie using Daniel's machine) Reopening and moving to Future.
Is this still requested? Note that the first WD of the CSS3 module: Border, did not include this.
I'll deal with this through the CSSWG if I still want it.