Because a web browser should be fundamentally neutral by default. All content should get rendered the same way, regardless of which server or domain it comes from.
That doesn't preclude the browser from making choices to protect privacy and security, but all sites should be treated equally (as Safari does with its tracking protection, for instance).
I don't think I agree with that. Do you want spam treated the same as your regular email for example? It is up to the implementation to decide what is {spy|mal|track|junk}ware.
The reverse is also problematic, sites treat browsers (or even user agents) very differently. Why should browsers not do the same?
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!
That doesn't preclude the browser from making choices to protect privacy and security, but all sites should be treated equally (as Safari does with its tracking protection, for instance).