I had found a workarround changing the default-application style:
div.tabs div.tab
{
position: relative;
background-color: #fff;
}
by this one...
div.tabs div.tab
{
position: relative;
background-color: #fff;
border: 1px solid #ffffff;
}
This fixes it indeed, but this yields an issue for firefox where the aligment seems to be one pixel off. For that reason I do not close this issue yet.