I saw some java code by Google using square bracket in comment. For example:
// [START initialize_auth]
mAuth = FirebaseAuth.getInstance();
// [END initialize_auth]
These snippets are linkers for the Firebase documentation.
You can click the class names on the Firebase site, and you will be taken to the corresponding block of the example code.
Hard to say if that is dynamically generated HTML, but there is no such special syntax for those comments in pure Java
Same question, but for iOS: What is this Objective-C/XCode annotation // [START_EXCLUDE]