如何强制执行select下拉选择

<select required>
  <option value="">请选择</option>
  <option value="option1">选项1</option>
  <option value="option2">选项2</option>
  <option value="option3">选项3</option>
</select>

<select id="mySelect">
  <option value="">请选择</option>
  <option value="option1">选项1</option>
  <option value="option2">选项2</option>
  <option value="option3">选项3</option>
</select>

<script>
  var select = document.getElementById("mySelect");
  select.addEventListener("change", function() {
    if (select.value === "") {
      alert("请选择一个选项");
    }
  });
</script>

<style>
  select:invalid {
    border: 1px solid red;
  }
</style>

<select required>
  <option value="">请选择</option>
  <option value="option1">选项1</option>
  <option value="option2">选项2</option>
  <option value="option3">选项3</option>
</select>