<howto-tabs> limit visible content by separating it into multiple panels. Only
one panel is visible at a time, while all corresponding tabs are always
visible. To switch from one panel to another, the corresponding tab has to be
selected.
By either clicking or by using the arrow keys the user can change the selection of the active tab.
If JavaScript is disabled, all panels are shown interleaved with the respective tabs. The tabs now function as headings.