Edited to explicitly mention muxup.com - you won't see any current articles using nested headings (i.e. h3 in addition to h1 for the article title and h2 for article headings) as every time I've used one I've not liked how hard it is to differentiate, and haven't invested the time in revisiting the CSS to fix it.
I just changed an H2 tag to an H3, and it looks like your H3 is way too small.
One thing I've seen for H3 and below is to use `font-style: italic` and font-weight 500 to distinguish it from the H2. I don't think that would work for your site though.
For your site, you could maybe throw a `color: #444` on H3 and below? You can also try `h1, h2 { margin-left: 1em }`.
Ah, that's because my static site generator only includes "needed" CSS directives, so if the page doesn't have h3 then this isn't included (I know, classic premature optimisation):
h3 {
font-size:2.074rem
}
Playing with font weights or color is a good suggestion, thanks.