This works fine:

<html-gl>
  <h1>Hello world</h1>
</html-gl>

This causes the h1 element to not render:

<html-gl>
  <h1 style="position:absolute">Hello world</h1>
</html-gl>

I wasn't able to determine if the positioning is just really offset to some absolute point and it's rendering off-canvas, or if it's not rendering at all.