In the Social Contract page at https://www.gentoo.org/get-started/philosophy/social-contract.html headings appear in normal font and with two hash signs in front, e.g. "##What is Gentoo?" for the first heading. In the HTML source it is: <p>##What is Gentoo?</p> As the Wayback Machine shows, it was still o.k. in June 2016: https://web.archive.org/web/20160609090140/https://www.gentoo.org/get-started/philosophy/social-contract.html
Looks like Jekyll was changed to require a blank after the # sign: https://talk.commonmark.org/t/heading-not-working/819
Thanks for the links! Page is fixed!
Fixed as of commit 53f997f98f04830bbe13687576caa57489e0e793.