<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>

在html5中，你可以使用完整的表达式:

<select required="required">

我不知道为什么简短的表达不管用，但是试试这个。 这就解决了。

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.

Works perfectly fine if the first option's value is null. Explanation : The HTML5 will read a null value on button submit. If not null (value attribute), the selected value is assumed not to be null hence the validation would have worked i.e by checking if there's been data in the option tag. Therefore it will not produce the validation method. However, i guess the other side becomes clear, if the value attribute is set to null ie (value = "" ), HTML5 will detect an empty value on the first or rather the default selected option thus giving out the validation message. Thanks for asking. Happy to help. Glad to know if i did.

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。

<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。)