> Then, however, some dipshit decides to extend the format by adding tags for things like bold, italics, underline etc. This is completely unnecessary for subtitles because the emphasis can be inferred from the dialogue.

Emphasis of an entire line can be inferred, but how can emphasis within a line be inferred when you don't know which utterances within the line correspond to which words in the subtitles (which, if you need subtitles because you don't know the language being spoken, you won't)?

While uncommon, I've occasionally seen font variants used for emphasis on professional subtitles for that reason.