For email, I agree some sort of spam filtering is necessary—but I wouldn't want an email client to alter the contents of messages depending on who the sender is. (By default at least—customization through plugins is great!)

> Sites treat browsers (or even user agents) very differently. Why should browsers not do the same?

But isn't that exactly why browsers are now phasing out user agents? I'm all for that—I shouldn't have to fake my user agent in order for Slack to work in a mobile browser.

If there's a certain browser feature that a website needs, and the website detects this feature isn't present and changes its behavior accordingly, that's quite different!