Currently, SE accepts HTML comments (e.g.
<!-- this is an HTML comment -->) as valid input for posts but they don't appear (since they are comments) this allows some users to get around the 30 character minimum limit for posts. It is even possible to make an empty post!
Some sites, like Stack Apps, use HTML comments for metadata about the post. Programming sites currently accept them to define the programming language, but as SE switches over to the new CommonMark standard, the language will have to be defined in a code fence.
So in all sites that don't accept metadata in comments, SE should render them as text to stop users from getting around the 30 character minimum limit.