如何才能使默认的下拉值不会在脚本中消失

<select>
  <option value="1" selected>默认选项1</option>
  <option value="2">选项2</option>
  <option value="3">选项3</option>
</select>

<select id="mySelect">
  <option value="1">默认选项1</option>
  <option value="2">选项2</option>
  <option value="3">选项3</option>
</select>

<script>
  window.onload = function() {
    document.getElementById("mySelect").value = "1";
  };
</script>

<select>
  <option value="1">默认选项1</option>
  <option value="2">选项2</option>
  <option value="3">选项3</option>
</select>

<style>
  option[value="1"] {
    selected: true;
  }
</style>