@Théry in the Obsidian CSS discord offered a little more refinement. I’m just copying it here because this is where people will find it. (I haven’t tested it yet)
For basic hiding of H6 in Preview:
.markdown-preview-view h6
{
display: none;
}
Refining margins with the embedded text:
h6 + p { margin-top: 0; }
h6 + ul { margin-top: 0; }