<a href="https://drafts.csswg.org/css-writing-modes/#text-combine-layout">9.1.2 Layout Rules</a> the spec says:
<blockquote>
The effective size of the composition is assumed to be 1em square; anything outside the square is not measured for layout purposes.
</blockquote>
<p>Hence, we need a glyph where:
<ul>
<li>its size is greater than 1em square
</ul>
<figure>
<img src="img/heavy_h.svg" width="200">
<!-- <img src="img/heavy_h_rotated.svg" width="200"> -->
<figcaption>The "Heavy H". The dashed region in the