> HTML vs plain text is a distraction; it's not the 1970's anymore and saving a byte or two here and there just isn't the computing world we're living in. Not when images, video, or audio files regularly get attached to email.It's not about HTML vs plain text. It's about HTML vs a way to style text which is actually useful to the users. HTML is so bad for this purpose, in practice 99% of HTML mails don't use any tags in their proper content and mails which could use some styling to make them more readable are using plain text, because HTML is of no help to them. My remark about overhead was actually about the quotes, whose sizes grow linearly as threads get longer (and the threads grow quadratically instead of linearly), not about HTML. You don't pay for tags you do not use (almost). The only rich HTML mails (as opposed to plain text HTML mails) I get are marketing mails, which want to track me and "stand out from the crowd", neither of which I have an interest in, and HR mails.
> What's more, how do you loop someone into a conversation they weren't previously a part of, if you don't have the history of the thread in each mail? This is something Slack gets right
Slack doesn't get it right. When you join a conversation, you always see the whole history. But in practice people conduct conversations according to the assumption of who is taking part in them at a given moment. They can't predict the future and that someone will add another person to the conversation, and now this person will see everything they wrote. There is no way to add them to the conversation with a context of "the last n messages" e.g. In an ideal mail client you would select which messages from the thread you want to share with the newly-added conversation participant.