Ah, that’s why, I forgot you were using relative font sizes. I’m afraid you’d need to use px instead of em, or add the following:
.cm-s-obsidian pre.HyperMD-header.HyperMD-header-1,
.cm-s-obsidian pre.HyperMD-header.HyperMD-header-2,
.cm-s-obsidian pre.HyperMD-header.HyperMD-header-3 {
font-size: inherit;
}
Again, it will be fixed in 0.7.5, but you can use the above solution as a temporary workaround.