默认情况下，您可以使用option元素的selected属性来选择一个选项。您可以使用select元素的必需属性来确保用户选择了一些东西。

在Javascript中，你可以检查selectedIndex属性来获得所选选项的索引，或者你可以检查value属性来获得所选选项的值。

根据HTML5规范，selectedIndex“返回第一个选中项的索引(如果有)，如果没有选中项则返回−1。value "返回第一个选中项的值(如果有)，如果没有选中项则返回空字符串。"如果selectedIndex = -1，那么你知道他们没有选择任何东西。

<button type="button" onclick="displaySelection()">What did I pick?</button>
<script>
    function displaySelection()
    {
        var mySelect = document.getElementById("someSelectElement");
        var mySelection = mySelect.selectedIndex;
        alert(mySelection);
    }
</script>

必填项:第一个值为空-必填项工作为空值

先决条件:正确的html5 DOCTYPE和命名输入字段

<select name="somename" required>
<option value="">Please select</option>
<option value="one">One</option>
</select>

根据文档(列表和粗体是我的)

The required attribute is a boolean attribute. When specified, the user will be required to select a value before submitting the form. If a select element has a required attribute specified, does not have a multiple attribute specified, and has a display size of 1 (do not have SIZE=2 or more - omit it if not needed); and if the value of the first option element in the select element's list of options (if any) is the empty string (i.e. present as value=""), and that option element's parent node is the select element (and not an optgroup element), then that option is the select element's placeholder label option.

是的，它正在工作:

<select name="somename" required>
     <option value="">Please select</option>
     <option value="one">One</option>
</select>

你得把第一选项空着。

试试这个

<select>
<option value="" style="display:none">Please select</option>
<option value="one">One</option>
</select>

<form action="">

<select required>

  <option selected disabled value="">choose</option>
  <option value="red">red</option>
  <option value="yellow">yellow</option>
  <option value="green">green</option>
  <option value="grey">grey</option>

</select>
<input type="submit">
</form>

<select>元素支持required属性，按照规范:

http://dev.w3.org/html5/spec-author-view/the-select-element.html#the-select-element

哪个浏览器不支持这个功能?

(当然，无论如何您都必须在服务器上验证，因为您不能保证用户将启用JavaScript。)