Using FontAwesome icons

<!-- Put them in the head tag -->
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css" />
<link rel="stylesheet" href="/vendors/@form-validation/umd/styles/index.min.css" />

<form id="demoForm">...</form>

<script src="/vendors/@form-validation/umd/bundle/popular.min.js"></script>

<script>
    document.addEventListener('DOMContentLoaded', function(e) {
        FormValidation.formValidation(
            document.getElementById('demoForm'),
            {
                fields: {
                    ...
                },
                plugins: {
                    icon: new FormValidation.plugins.Icon({
                        valid: 'fa fa-check',
                        invalid: 'fa fa-times',
                        validating: 'fa fa-refresh',
                    }),
                },
            }
        );
    });
</script>