Adding a class of nav-tabs--active will set a nav tab item to active.

By default nav-tabs__content items are hidden. You must give at least one content area the class nav-tabs__content--active otherwise all tab content areas will be hidden.

Tabs can toggle content area visiblity by giving a content area an id. Placing this id in the href attribute of a tab item preceeded by a # will allow the tab to show that content area when clicked.