The "send" button. Simple and useful.

It is enough just to improve the usability of the form in a simple but extremely effective way - deactivating the “send” button after pressing it.

In order not to be unfounded, I will give an example for jQuery:

$('#id_').click(function()
{
$('this').attr('disabled', 'disabled')
}
);

Ideally, the button should become active only after the form is fully valid.
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As the nooze prompted Habrapheater, it’s worth considering that in the case of AJAX response to sending, you should take into account errors (for example, a username and password that do not match, or a request timeout)

PS> Perhaps, the topic has already been raised, but I did not find any references to it.
PPS> Thank you for shitting karma, you are good people :)
PPPS> I do not accept the model Habra, in which the author puts everything and everyone in the top, in this case there is simply nothing to discuss.