@dantleech the `%` is a reserved character in Yaml. That means you can only use them if you put quotes around it. The `%` isn't used in Yaml 1.1, that's why there are no parsing errors. However, there are 2 reasons we should do it this way:

 1. We should not use invalid things in the documentation
 2. The highlighting only works with valid code blocks. (see also https://github.com/symfony/symfony-docs/issues/1886#issuecomment-14035875 )