wichita.edu
… represent hierarchical structure (in html, <h1>, <h2>, <h3>, <h4>, <h5>, <h6>). It's just like a taxonomy in biology would go domain, kingdom, phylum, class, order, family, genus and then species. Only even easier to remember, because they're numbered. And just like a species can't be named without a genus, an h6 must have an h5 above it. In the hierarchy…
https://www.wichita.edu/services/mrc/Web_Services/announcements/2019-09-25-accessibility.php