Can't make the validation work in Bootstrap 4

Validation has changed as of the Bootstrap 4 beta release.

The valid state selectors use the was-validated class which would be added dynamically after validating the form via client-side JavaScript. For example...

<form class="container was-validated" novalidate="">
    <div class="form-group">
        <label class="form-control-label" for="inputSuccess1">Input with success</label>
        <input type="text" class="form-control" name="i1" id="inputSuccess1">
        <div class="valid-feedback">Success! You've done it.</div>
    </div>
    <div class="form-group">
        <label class="form-control-label" for="inputSuccess2">Input with danger</label>
        <input type="text" class="form-control" name="i2" required="" id="inputSuccess2">
        <div class="invalid-feedback">That didn't work.</div>
    </div>
    <div class="">
        <button type="submit" class="btn btn-secondary">Text</button>
    </div>
</form>

https://www.codeply.com/go/45rU7UOhFo

Update 2019 - Bootstrap 4.0.0
Form Validation Example Demo

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As explained in the docs, if you intend to use server-side validation you can simply set the is-valid or is-invalid classes on the form-controls...

<form class="container">
    <div class="form-group">
        <label class="form-control-label" for="inputSuccess1">Input with success</label>
        <input type="text" class="form-control is-valid" id="inputSuccess1">
        <div class="valid-feedback">Success! You've done it.</div>
    </div>
</form>

It appears the validation changes again in the final release version of Bootstrap 4: http://getbootstrap.com/docs/4.0/components/forms/#validation.

It becomes more complicated than I thought.

Custom style client side validation is recommended:

1. When validated, the form adds a class named was-validated.
2. Feedback messages are wrapped within .valid-feedback or .invalid-feedback.

For server-side validation:

1. No need for was-validated class on the <form> tag.
2. Add .is-valid or .is-invalid on the input control.
3. Add .invalid-feedback or .valid-feedback for the feedback message.