When you post a comment on HN, the server inserts HTML tags into your input. Isn't that essentially the same thing?

No, because there is a clear separation between the content and the envelop. You wouldnt expect the post office to open your physical letters and write routing instructions to the postmen for delivery

But I agree with sibling comment: it makes more sense when its called "encoding" instead of "inserting chars into original stream"

	▲	1718627440 3 hours ago \| parent [-]
		> You wouldnt expect the post office to open your physical letters and write routing instructions to the postmen for delivery Digital communication is based on the postmen reading, transcribing and copying your letters. There is a reason why digital communication is treated differently then letters by the law and why the legally mandated secrecy for letters doesn't apply to emails.