It is possible to add validation on the HTML code to help the user to complete the form. For example, adding the required attribute to a field will return a message when submitting the form if that field is left blank. You can also use CSS to display the invalid fields differently.
<form action="https://app.cyberimpact.com/optin" method="post" accept-charset="utf-8">
...
<label for="ci_firstname">First Name</label>
<input type="text" id="ci_firstname" name="ci_firstname" maxlength="255" required />
</form>
<style>
input:invalid { border: 2px dashed red; }
</style>
You'll find a more complete reference on the Mozilla website. Note that this method can be overridden, but it usually gives good results.