<html>

<body>
  <input type="text" class="required email">
  <button onclick="validateForm()">Submit</button>

</body>

</html>
function validateForm() {
  var errorMessage = "The following errors were found:\n";
  var elements = document.getElementsByTagName("input");
  for (let i = 0; i < elements.length; i++) {
    if (elements[i].className.includes("required")) {
      if (!elements[i].value) {
        errorMessage += "Missing required input\n";
      }
    }

    if (elements[i].className.includes("email")) {
      if (!elements[i].value.includes("@")) {
        errorMessage += "Invalid Email\n";
      }
    }
  }

  alert(errorMessage);
}
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