JS multi step form

//js multi steps form
//HTML
<form id="regForm" action="">

    <h1>Register:</h1>

    <!-- One "tab" for each step in the form: -->
    <div class="tab">Name:
      <p><input placeholder="First name..." oninput="this.className = ''"></p>
      <p><input placeholder="Last name..." oninput="this.className = ''"></p>
    </div>

    <div class="tab">Contact Info:
      <p><input placeholder="E-mail..." oninput="this.className = ''"></p>
      <p><input placeholder="Phone..." oninput="this.className = ''"></p>
    </div>

    <div class="tab">Birthday:
      <p><input placeholder="dd" oninput="this.className = ''"></p>
      <p><input placeholder="mm" oninput="this.className = ''"></p>
      <p><input placeholder="yyyy" oninput="this.className = ''"></p>
    </div>

    <div class="tab">Login Info:
      <p><input placeholder="Username..." oninput="this.className = ''"></p>
      <p><input placeholder="Password..." oninput="this.className = ''"></p>
    </div>

    <div style="overflow:auto;">
      <div style="float:right;">
        <button type="button" id="prevBtn" onclick="nextPrev(-1)">Previous</button>
        <button type="button" id="nextBtn" onclick="nextPrev(1)">Next</button>
      </div>
    </div>
    </form>

//css
/* Hide all steps by default: */
.tab {
  display: none;
}

//js
var currentTab = 0;
showTab(currentTab);

function showTab(n) {
  var x = document.getElementsByClassName("tab");
  x[n].style.display = "block";
  if (n == 0) {
    document.getElementById("prevBtn").style.display = "none";
  } else {
    document.getElementById("prevBtn").style.display = "inline";
  }
  if (n == (x.length - 1)) {
    document.getElementById("nextBtn").innerHTML = "Submit";
  } else {
    document.getElementById("nextBtn").innerHTML = "Next";
  }
}

function nextPrev(n) {
  var x = document.getElementsByClassName("tab");
  if (n == 1 && !validateForm()) return false;
  x[currentTab].style.display = "none";
  currentTab = currentTab + n;
  if (currentTab >= x.length) {
    document.getElementById("regForm").submit();
    return false;
  }
  showTab(currentTab);
}

function validateForm() {
  var x, y, i, valid = true;
  x = document.getElementsByClassName("tab");
  y = x[currentTab].getElementsByTagName("input");
  for (i = 0; i < y.length; i++) {
    if (y[i].value == "") {
      y[i].className += " invalid";
      valid = false;
    }
  }
  return valid;
}