In version 6, Font Awesome changed so that the class names look like `fa-solid fa-cat` instead of `fas fa-cat`. mdBook is not handling the new style.
This is a test for when an icon is missing.
This example was intended to demonstrate that it only translates if there are no child elements.
This adds a bunch of tests to better exercise the HTML rendering and to be able to track any changes in its behavior. This includes a new `check_all_main_files` to more conveniently check the HTML content of every chapter in a book.
