Why does HTML5 form-validation allow emails without a dot?

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.

Because a@b is a valid email address (eg localhost is a valid domain). See http://en.wikipedia.org/wiki/Email_address#Examples

Also, keep in mind that you should always do the input validation in server. The client side validation should be only for giving feedback to the user and not be relied on, since it can be easily bypassed.