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Based on the comment discussion, I can see no reason to consider support for syntax highlight hinting via HTML comments. "Code fences" have a built-in syntax highlighting suggestion and are all around quite practical and standard: such "fenced code blocks" [are part of the CommonMark standard](https://spec.commonmark.org/0.31.2/#indented-code-blocks), and overall easier to work with than "indented code blocks". They're also, for example, the [recommended way to do it](https://discuss.python.org/t/about-the-python-help-category/42) on the Python Discourse forum.

However, the warning about HTML comments definitely should be suppressed, IMO. First off, there's no good reason to treat comments as something that need to be "filtered out". For the most "important" Q&A - ones that are expected to be viewed the most often in the long run, or used to close many duplicates - they can offer important information for future editors about why a given detail was formatted in a specific way. (While some aspects of this might be better discussed in comments, surely this isn't universal.)

Second, it doesn't make sense to give the user a "warning" about HTML comments being stripped from the resulting post, because *that would be expected anyway* - the purpose of a comment is exactly to avoid actually changing anything while providing metadata. It's especially user-unfriendly to see `<undefined>` in the warning message - it's confusing, since the user didn't actually write a tag with that name; and it generally comes across as though the system encountered some problem while trying to format the message.

On the other hand, it probably doesn't matter if the system *actually removes* the HTML comments from the final rendered HTML. Just as long as it isn't treated like a cryptic and spurious error.