Form validation. Big deal. But not so much for this style, because it uses all the functionallity from bootstrap form validation.
Hey, it's not always bootstraps validation workes out in your project. Sometimes you validate the forms at the server.
Make sure to send back class server-side-val__error to your input, select or textarea.
If you want, you can also add a div with the class of server-side-val__feedback. Just type your form feedback right inside the element.
To futher explain what happened, you can add either a error-toast, warning-toast or a success-toast, depending on if it went good or bad.