Frontends can just support both. In Mastodon, I can search for either `@username@example.com` or `https://example.com/@username` and both will suffice to show the same result.

I prefer the email format, like username@example.com

Double @ just look so ugly.

The problem with that format is that it had better also work as an email address, because otherwise people will be extremely confused by it.

> better also work as an email address

Sure why not? Make ActivityPub work over SMTP doesn't sounds like a bad idea.

That would imply that's an email, when in fact it's just a fedi account. It's there for a reason, another one (besides accessibility) is to make it easier parsing for other instances.