You can theoretically have an address without a “.” in.
Since technically things such as:
user@com
user@localserver
user@[IPv6:2001:db8::1]
Are all valid emails.
So the standard HTML5 validation allows for all valid E-mails, including the uncommon ones.
For some easy to read explanations (Instead of reading through the standards):
http://en.wikipedia.org/wiki/Email_address#Examples
Update from a comment: ICANN banned so-called “dotless” domains in 2013, but since that doesn’t affect every case listed above, allowing “dotless” addresses is still valid.