Can't make the validation work in Bootstrap 4

Question

I\'m trying to implement validation by Bootstrap and I\'ve pasted the following sample on my page:

Answer 1

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
   tag.
2. Add .is-valid or .is-invalid on the input control.
3. Add .invalid-feedback or .valid-feedback for the feedback message.