The <section> element provides a flex column layout with a small gap.
The <menu> element creates a horizontal nav bar (accepts the .active and
.disabled classes):
Typical page structure:
<body>
<div id="root">
<menu>...</menu>
<main>
<fieldset>...</fieldset>
</main>
</div>
</body>
The #root container provides the central 1-column bordered box layout with
1200px width (it can be used as the React root container, for example).