Prism does its best to encourage good authoring practices. Therefore, it only works with `undefined
If you use that pattern, the <pre> will automatically get the language-xxxx class (if it doesn’t already have it) and will be styled as a code block.
If you want to prevent any elements from being automatically highlighted, you can use the attribute data-manual on the
<script> element you used for prism and use the API. Example:
<script src="prism.js" data-manual></script>