pre>code should have `font-family: monospace` fallback
The styles for code blocks have a lot of fonts in their
font-family list, but unfortunately, none of those are installed on my Ubuntu installation. Whether or not you choose to add a font that Ubuntu systems do have to that list (such as Liberation Mono or Ubuntu Mono), one thing that should certainly be done is adding
monospace as the last font in the list as a fallback, so that on systems like mine users do not have to see serif fonts in the code blocks.