Slot fallbacks

A component can specify fallbacks for any slots that are left empty, by putting content inside the <slot> element:

<div class="box">
    <slot>
        <em>no content was provided</em>
    </slot>
</div>

We can now create instances of <Box> without any children:

<Box>
    <h2>Hello!</h2>
    <p>This is a box. It can contain anything.</p>
</Box>
<Box/>