Missing https://github.com/sindresorhus/github-markdown-css

[javajosh]

Hi there, I think that github's own css is worth considering as a starting point for semantic css. There exists one at https://github.com/sindresorhus/github-markdown-css. It's only 5.8kB transfer, without minification, so I think it deserves inclusion

[denisinvader]

But it required .markdown-body class usage actually 🤓