optionally accept specially labelled children that will appear in named "slots".
<!-- load web component module -->
<script type="module" src="components/search-box.js">
<!-- use its tag in HTML -->
<search-box placeholder="Search..."></search-box>
<!-- using named slots -->
<search-box>
<img src="images/search-icon.png" slot="left"/>
<img src="images/cancel-icon.png" slot="right"/>
</search-box>