I do not understand what the function of the pseudo-classes CSS "top", "right", "bottom" and "left" in a tab-pane.
They are related to the side property.
The side property relates to where the tabs are placed in relation to the visible tab pane.
If you are not using that property, you don't need to worry about them to much.
The default value for side is top, so that is the default pseudo-class which will be active.
Another thing ... How to center the tabs in a tab-pane?