以下是JavaScript的表单验证功能，可根据JS代码编写出你想要的HTML和CSS的代码。

//1.声明变量
var emailObj;
var usernameObj;
var passwordObj;
var confirmObj;
var emailMsg;
var usernameMsg;
var passwordMsg;
var confirmMsg;
//页面加载之后，获取页面中的对象
window.onload = function() {
  emailObj = document.getElementById("email");
  usernameObj = document.getElementById("username");
  passwordObj = document.getElementById("password");
  confirmObj = document.getElementById("confirm");
  emailMsg = document.getElementById("emaileMsg");
  usernameMsg = document.getElementById("usernameMsg");
  confirmMsg = document.getElementById("confirmMsg");
};
//3.检验整个表单
function checkForm() {
  var bEmail = checkEmail();
  var bUsername = checkUsername();
  var bPassword = checkPassword();
  var bConfirm = checkCondfirm();
  return bUsername && bPassword && bConfirm && bEmail;
}
//4.验证邮箱
function checkEmail() {
  var regex = /^([a-zA-Z]|[0-9])(\w|\-)+@[a-zA-Z0-9]+\.([a-zA-Z]{2,4})$/;
  var value = emailObj.value;
  var msg = "";
  if(!value) {
    msg = "邮箱必须填写";
  } else if (!regex.test(value)) {
    msg = "邮箱格式不合法";
  }
  emailMsg.innerHTML = msg;
  emailObj.parentNode.parentNode.style.color = msg == ""?"black":"red";
  return msg == "";
}
//5.验证用户名‘
function checkUsername() {
  var regex = /^[a-zA-Z0-9_-]\w{0,9}$/;  //字母数字下划线1-10位，不能是数字开头
  var value = usernameObj.value;     //获取usernameObj中的文本
  var msg = "";              //最后的提示小事，默认为空
  //如果用户名是null或"",!value的值为false，如果不为空 !value值为true
  if(!value) {
    msg = "用户名必须填写";
  } else if (regex.test(value)) {
    msg = "用户名不合法";
  }
  usernameMsg.innerHTML = msg;
  usernameObj.parentNode.parentNode.style.color = msg == ""?"black":"red";
  return msg == "";            //如果提示消息为空则代表没出错，返回true
}
//6.验证密码
function checkPassword() {
  var regex = /^.{6,16}$/;      //任意字符，6-16位
  var value = passwordObj.value;
  var msg = "";
  if(!value) {
    msg = "密码必须填写";
  } else if (!regex.test(value)) {
    msg = "密码不合法";
  }
  passwordMsg.parentNode.parentNode.style.color = msg == ""?"black":"red";
  return msg == "";
}
//7.验证确认密码
function checkCondfirm() {
  var passwordValue = passwordObj.value;
  var confirmValue = confirmObj.value;
  var msg = "";
  if(!confirmValue) {
    msg = "确认密码填写";
  } else if (passwordValue != confirmValue) {
    msg = "密码必须保持一致";
  confirmMsg.innerHTML = msg;
  confirmObj.parentNode.parentNode.style.color = msg == ""?"black":"red";
  return msg == "";
}

以下代码是简单的测试邮箱是否匹配成功的代码，可根据下面来实现具体的逻辑