Может будет достаточно required?
<form onsubmit="return myForm()" id="testForm">
<div>
<label for="firstName">First name</label>
<input type="text" id="firstName" required/>
<div id='err_fio1' class='error'></div>
</div>
<div>
<label for="lastName">Last name</label>
<input type="text" id="lastName" required/>
<div id='err_fio2' class='error'></div>
</div>
<div>
<label for="email">Email</label>
<input type="text" id="email" required/>
<div id='err_fio3' class='error'></div>
</div>
<button type="submit">Send</button>
</form>