JSFiddle - React, Tailwind, and code Playground

HTML

<form method="post" id="form_top" name="form_top" action="/mail.php">
    <input type="text" class="textbox" name="fio" id="fio" placeholder="NAME"><div id="fio_err"></div><br><br>
    <input type="text" class="textbox" name="tel" id="_phone_top" placeholder="PHONE"><div id="tel_err"></div><br><br>
    <input type="text" class="textbox" name="email" id="email" placeholder="E-Mail"><div id="email_err"></div><br><br>
    <input type="text" class="textbox" name="company" placeholder="COMP"><br><br>
    <input type="text" name="formfrom" value="верхняя форма" hidden>
    <input type="submit" class="textbox" name="ok" id="ok" value="Submit!" disabled><br>
</form>

JavaScript

function checkForm() {

    var err = 0;

    if (document.getElementById("fio").value == '') {

    document.getElementById("fio").style.border = "1px solid #FF0000";
    document.getElementById("fio_err").innerHTML = "Enter data";
    err = 1;

}

if (document.getElementById("_phone_top").value.length < 5) {

document.getElementById("_phone_top").style.border = "1px solid #FF0000";
document.getElementById("tel_err").innerHTML = "length < 5";
err = 1;

}

var email = document.getElementById('email').value;
emailTest = "^[_.0-9a-z-]+@([0-9a-z][0-9a-z_-]+.)+[a-z]{2,4}$";
var regex = new RegExp(emailTest); 

if (!regex.test(email)) {

document.getElementById("email").style.border = "1px solid #FF0000";
document.getElementById("email_err").innerHTML = "invalid e-mail";
err = 1;

}

    if ( err == 1 ) {
       document.getElementById('ok').setAttribute("disabled","disabled");
    } else {
       document.getElementById('ok').removeAttribute("disabled");
    }

// if (err == 1) document.getElementById("ok").disabled = true; 
// if (err == 0) document.getElementById("ok").enable = false; 

}

function setBorder(id) {

document.getElementById(id).style.border = "1px solid #C0C0C0";
document.getElementById(id+"_err").innerHTML = "";

}

document.getElementById('ok').addEventListener("mouseover", checkForm);