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.

Try adding this to the input

pattern="[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,63}$"

Fiddle

The RFC 822, chapter 6, gives the specification of an address in augmented Backus-Naur Form (BNF):

addr-spec   =  local-part "@" domain
local-part  =  word *("." word)
domain      =  sub-domain *("." sub-domain)

Using this specification a@b is a valid address.

UPDATE

To answer the comment of Trejkaz, I add the following definitions. We see that SPACE are allowed but only in quoted string.

word          =  atom / quoted-string
atom          =  1*<any CHAR except specials, SPACE and CTLs>
quoted-string = <"> *(qtext/quoted-pair) <">
SPACE         =  <ASCII SP, space>
CTL           =  <any ASCII control character and DEL> 
qtext         =  <any CHAR excepting <">, "\" & CR, and including linear-white-space>
quoted-pair   =  "\" CHAR  

This MDN page shows the regex browsers should use to validate the email:

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/email#Validation

You can slightly change this regex to require at least one dot in the domain name: change the star * at the end of the regex to a plus +. Then use that regex as the pattern attribute:

You can customize the pattern of the email field:

input:valid { border-color: green } input:invalid { border-color: red } Email:
Non-dots Email:

Here is how you can do it with html5 using regex pattern. You can also include a custom message to display.

Hostnames without a TLD appear to be valid.

I say "appear" because there is this 2013 ICANN prohibition on dotless domains . . .

At its meeting on 13 August 2013, the ICANN Board New gTLD Program Committee (NGPC) adopted a resolution affirming that "dotless domain names" are prohibited.

. . . but judging from real world experience, it appears to have never been enforced.

Regardless, the PHP function FILTER_VALIDATE_EMAIL doesn't allow for dotless domain names.

So here's a simple back-end validation set-up that covers both your email and required fields:

if (empty($required_field) OR empty($another_required_field) OR
    !filter_var($email_field, FILTER_VALIDATE_EMAIL)) {
    // error handling here
    exit;
    }

While the "malformed" email may get passed the browser, it won't get passed the server.

References:

FILTER_VALIDATE_EMAIL definition

List of valid and invalid email addresses

This pattern always works for me.

Text must in lowercase pattern="[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,}$" but I think it covers more or less most emails.