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Could someone explain why they're hated? I don't know much about E3, but used them a few times without issue
I think it's similar to PHP, they're criticized by the vocal newbies who are chasing the newest and shiniest. For me personally, w3schools has been one of the most reliable sources of reliable solutions that work not just for the latest Chrome build but across many different browsers and platforms. Given the cumulative nature of HTML, "outdated" is another word for "reliable and established", IMO.
They're better today, but it's still... let's say an equivalent of empty calories instead of a healthy MDN meal.