Kudo 致@enxaneta,答案是:
<div>
<svg width="64" height="64" fill="currentColor">
<!-- 100% -->
<use href="#bi-heart" width="64" height="64"></use>
<!-- 50%, centered -->
<use href="#bi-stag" width="32" height="32" x="16" y="16"></use>
</svg>
</div>