When requested via HTTPS the gentoo wiki still contains content fetched via HTTP. Reason for that is the piwik tracker code at the end of the site: The noscript element contains an image loaded from http://piwik.gentoo.org/piwik.php?idsite=1 As a workaround you could change the URL to //piwik.gentoo.org/piwik.php?idsite=1
Fixed in the codebase. Might take a while for the cache to clear on all pages.