Word Press Contact Form 7 Validation Not Working : I am using class:required on my form fields but the validation is not working.

I would appreciate for help and guidance. Thank you!

<div class="request-form animation" data-animation="fadeInRight">
<h4>REQUEST AN ESTIMATE</h4>
<div style="margin-bottom: 18px">Our estimates are free, professional and fair. Most estimates require a site visit. </div>
[text Full Name class:input-custom class:input-full placeholder class:required "Name:"]
[text Phone class:input-custom class:input-full placeholder class:required "Phone:"]
[text E-mail class:input-custom class:input-full placeholder class:required "Email:"]
[textarea Message class:input-custom class:input-full placeholder class:required "Message:"]
[submit class:btn class:btn-light "SEND MESSAGE"]

</div>