Trigger autocomplete without submitting a form

Tested with Chrome, IE and Firefox:

<iframe id="remember" name="remember" class="hidden" src="/content/blank"></iframe>

<form target="remember" method="post" action="/content/blank">
  <fieldset>
    <label for="username">Username</label>
    <input type="text" name="username" id="username" value="">
    <label for="password">Password</label>
    <input type="password" name="password" id="password" value="">
  </fieldset>
  <button type="submit" class="hidden"></button>
</form>

In your Javascript trigger the submit, e.g. $("form").submit(); $("#submit_button").click() (updated from comments)

You need to return an empty page at /content/blank for get & post (about:blank didn't work for me but YMMV).

I haven't tested this, but it might work if you submit the form to a hidden iframe (so that the form is actually submitted but the current page is not reloaded).

<iframe name="my_iframe" src="about:blank"></iframe>

<form target="my_iframe" action="about:blank" method="get">...</form>

---WITHOUT IFRAME---

Instead of using iframe, you can use action="javascript:void(0)", this way it doesn't go to another page and autocomplete will store the values.

<form action="javascript:void(0)">
    <input type="text" name="firstName" />
    <button type="submit">Submit</button>
</form>

We know that the browser saves its information only when the form is submitted, which means that we can't cancel it with return false or e.preventDefault()

What we can do is make it submit the data to nowhere without reloading a page. We can do that with an iframe

<iframe name="💾" style="display:none" src="about:blank"></iframe>

<form target="💾" action="about:blank">
    <input name="user">
    <input name="password" type="password">
    <input value="Login" type="submit">
</form>

Demo on JSfiddle (tested in IE9, Firefox, Chrome)

Pros over the currently accepted answer:

• shorter code;
• no jQuery;
• no server-side page loaded;
• no additional javascript;
• no additional classes necessary.

There is no additional javascript. You normally attach an handler to the submit event of the form to send the XHR and don't cancel it.

Javascript example

// for modern browsers with window.fetch
document.forms[0].addEventListener('submit', function (event) {
    fetch('login.php', {
        method: 'post',
        body: new FormData(event.target))
    }).then(r => r.text()).then(() => { /* login completed */ })
    // no return false!!
});

No-javascript support

Ideally, you should let the form work without javascript too, so remove the target and set the action to a page that will receive your form data.

<form action="login.php">

And then simply add it via javascript when you add the submit event:

formElement.target = '💾';
formElement.action = 'about:blank';