A cursory glance points at RenderBoxModelObject::paintBoxShadow() which uses the values from RenderStyle::getBorderRadiiForRect() for the shadow as well. If the border-radius is specified in percent, those values should be relative to the shadow's size (with spread taken into account), not the element casting the shadow.
Screenshots from WebKit and Firefox4 coming..
Created attachment 74559 [details]
Simple page illustrating the problem
Created attachment 74560 [details]
WebKit's rendering of test page
Created attachment 74561 [details]
Firefox4's rendering of test page
Created attachment 77400 [details]
repro with px.
This isn't limited for percentage value. other values are should be also scaled accordingly
because box-radius is for "box", not shadow.
Mozilla and Opera do scale.
Ah, I realized that this is a dup of Bug 49726
*** This bug has been marked as a duplicate of bug 49726 ***
Bug 27417 and LayoutTests/fast/box-shadow/spread.html says this is intentional.
But I don't think so.