The styling for li:last-child:after is ignored by WebKit (nightly build 30153) if more than one li exists within the ul. Please see the attached file for an example.
Created attachment 19086 [details] Sample List
The problem goes away if you add a dummy :first-child rule: li:first-child {} So perhaps the problem is that the "affected by" bits are not set correctly.
<rdar://problem/5736953>
Created attachment 19087 [details] Call setUsesSiblingRules(true) for all CSS3 selectors that require it
Comment on attachment 19087 [details] Call setUsesSiblingRules(true) for all CSS3 selectors that require it r=me
Fixed in <http://trac.webkit.org/projects/webkit/changeset/30162>.