如果表单未填写，则禁用提交按钮并更改按钮的CSS

<form>
  <input type="text" id="name" placeholder="姓名">
  <input type="email" id="email" placeholder="邮箱">
  <button type="submit" id="submit-btn">提交</button>
</form>

#submit-btn {
  background-color: #4CAF50;
  color: white;
  padding: 10px 20px;
  border: none;
  cursor: pointer;
}

#submit-btn:disabled {
  background-color: gray;
  cursor: not-allowed;
}

const nameInput = document.getElementById('name');
const emailInput = document.getElementById('email');
const submitBtn = document.getElementById('submit-btn');

function checkForm() {
  if (nameInput.value !== '' && emailInput.value !== '') {
    submitBtn.disabled = false;
  } else {
    submitBtn.disabled = true;
  }
}

nameInput.addEventListener('input', checkForm);
emailInput.addEventListener('input', checkForm);