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.

<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>

将选择框的第一项的值设为空白。

所以当你发布表单时，你会得到空白值，使用这种方式，你会知道用户没有从下拉菜单中选择任何东西。

<select name="user_role" required>
    <option value="">-Select-</option>
    <option value="User">User</option>
    <option value="Admin">Admin</option>
</select>

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.

首先，您必须在第一个选项中分配空白值。 选择这里。而不是只需要工作。

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。