Multi Step Form with Progress Bar using JavaScript and CSS3

var currentTab = 0; // Current tab is set to be the first tab (0)
        showTab(currentTab); // Display the current tab
        
        function showTab(n) {
          // This function will display the specified tab of the form...
          var x = document.getElementsByClassName("step");
          x[n].style.display = "block";
          //... and fix the Previous/Next buttons:
          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";
          }
          //... and run a function that will display the correct step indicator:
          fixStepIndicator(n)
        }
        
        function nextPrev(n) {
          // This function will figure out which tab to display
          var x = document.getElementsByClassName("step");
          // Exit the function if any field in the current tab is invalid:
          if (n == 1 && !validateForm()) return false;
          // Hide the current tab:
          x[currentTab].style.display = "none";
          // Increase or decrease the current tab by 1:
          currentTab = currentTab + n;
          // if you have reached the end of the form...
          if (currentTab >= x.length) {
            // ... the form gets submitted:
            document.getElementById("signUpForm").submit();
            return false;
          }
          // Otherwise, display the correct tab:
          showTab(currentTab);
        }
        
        function validateForm() {
          // This function deals with validation of the form fields
          var x, y, i, valid = true;
          x = document.getElementsByClassName("step");
          y = x[currentTab].getElementsByTagName("input");
          // A loop that checks every input field in the current tab:
          for (i = 0; i < y.length; i++) {
            // If a field is empty...
            if (y[i].value == "") {
              // add an "invalid" class to the field:
              y[i].className += " invalid";
              // and set the current valid status to false
              valid = false;
            }
          }
          // If the valid status is true, mark the step as finished and valid:
          if (valid) {
            document.getElementsByClassName("stepIndicator")[currentTab].className += " finish";
          }
          return valid; // return the valid status
        }
        
        function fixStepIndicator(n) {
          // This function removes the "active" class of all steps...
          var i, x = document.getElementsByClassName("stepIndicator");
          for (i = 0; i < x.length; i++) {
            x[i].className = x[i].className.replace(" active", "");
          }
          //... and adds the "active" class on the current step:
          x[n].className += " active";
        }

* {
            box-sizing: border-box;
        }
        body{
            font-size: 1em;
            font-family: 'Open Sans', sans-serif;
        }
        h1{
            text-align: center;
            font-size: 1.5em;
        }
        #signUpForm {
            max-width: 500px;
            background-color: #ffffff;
            margin: 40px auto;
            padding: 40px;
            box-shadow: 0px 6px 18px rgb(0 0 0 / 9%);
            border-radius: 12px;
        }
        #signUpForm .form-header {
            display: flex;
            gap: 5px;
            text-align: center;
            font-size: .9em;
        }
        #signUpForm .form-header .stepIndicator {
            position: relative;
            flex: 1;
            padding-bottom: 30px;
        }
        #signUpForm .form-header .stepIndicator.active {
            font-weight: 600;
        }
        #signUpForm .form-header .stepIndicator.finish {
            font-weight: 600;
            color: #009688;
        }
        #signUpForm .form-header .stepIndicator::before {
            content: "";
            position: absolute;
            left: 50%;
            bottom: 0;
            transform: translateX(-50%);
            z-index: 9;
            width: 15px;
            height: 15px;
            background-color: #d5efed;
            border-radius: 50%;
            border: 3px solid #ecf5f4;
        }
        #signUpForm .form-header .stepIndicator.active::before {
            background-color: #a7ede8;
            border: 3px solid #d5f9f6;
        }
        #signUpForm .form-header .stepIndicator.finish::before {
            background-color: #009688;
            border: 3px solid #b7e1dd;
        }
        #signUpForm .form-header .stepIndicator::after {
            content: "";
            position: absolute;
            left: 50%;
            bottom: 9px;
            width: 100%;
            height: 3px;
            background-color: #f3f3f3;
        }
        #signUpForm .form-header .stepIndicator.active::after {
            background-color: #a7ede8;
        }
        #signUpForm .form-header .stepIndicator.finish::after {
            background-color: #009688;
        }
        #signUpForm .form-header .stepIndicator:last-child:after {
            display: none;
        }

        #signUpForm p{
            text-align: center;
            margin-bottom: 25px;
        }
        .mb-15{
            margin-bottom: 15px;
        }
        
        #signUpForm input {
            padding: 15px 20px;
            width: 100%;
            font-size: 1em;
            border: 1px solid #e3e3e3;
            border-radius: 5px;
        }
        #signUpForm input:focus {
            border: 1px solid #009688;
            outline: 0;
        }
        #signUpForm input.invalid {
            border: 1px solid #ffaba5;
        }
        #signUpForm .step {
          display: none;
        }
        #signUpForm .form-footer{
            overflow:auto;
            display: flex;
            gap: 20px;
        }
        #signUpForm .form-footer button{
            background-color: #009688;
            border: 1px solid #009688 !important;
            color: #ffffff;
            border: none;
            padding: 13px 30px;
            font-size: 1em;
            cursor: pointer;
            border-radius: 5px;
            flex: 1;
            margin-top: 5px;
        }
        #signUpForm .form-footer button:hover {
          opacity: 0.8;
        }
        
        #signUpForm .form-footer #prevBtn {
            background-color: #fff;
            color: #009688;
        }