Well, as far as the docs at http://commonmark.org/help go, per the CSS and HTML on that page, it’s Google’s open source Roboto font, freely available on the web, feel free to look it up. It is the monospace variant, but exact same family, so Roboto Mono.
Here the font stack (for a code block) is
font-family: Consolas, Menlo, Monaco, "Lucida Console", "Liberation Mono", "DejaVu Sans Mono", "Bitstream Vera Sans Mono", "Courier New", monospace;
Which, on my machine, would be Consolas. Using Google/Roboto is easier since every browser can download and use the same font, at the cost of some network transfer delay, versus the “native” OS fonts.