Here is a picture for reference. I don't want there to be a small gap underneath the tab. I've made sure that the containing table had cellspacing/padding = "0px" but still it persists.

Hi Sabrick,

Please move your TabContainer into Panel, then add the Panel to your RoundedCornersExtender.  It should work.  If it doesn't, please add a css to the newly added div.  For example.

.divContainer{

  clear: all;

  //your css here. 

}

Best regards,

Jonathan