Document tabs have border between tab label and content

The tab content wasn't using position: relative before this change but the tab label was. Therefore, the tab label was in front of the tab content as all positioned elements are above non-positioned elements, and it was hiding the border. Now, the tab content is also relatively positioned, meaning that is now above the tab label (and thus the border is visible) as the default stacking order within positioned elements follows the source order. See also https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_positioned_layout/Stacking_without_z-index