HTML 属性：required

布尔值 required 属性（如果存在）指示用户必须在提交所属表单之前指定输入值。

¥The Boolean required attribute, if present, indicates that the user must specify a value for the input before the owning form can be submitted.

text、search、url、tel、email、password、date、month、week、time、datetime-local、number、checkbox、radio、file、<input> 类型以及 <select> 和 <textarea> 表单控制元素支持 required 属性。如果出现在任何这些输入类型和元素上，则 :required 伪类将匹配。如果不包含该属性，则 :optional 伪类将匹配。

¥The required attribute is supported by text, search, url, tel, email, password, date, month, week, time, datetime-local, number, checkbox, radio, file, <input> types along with the <select> and <textarea> form control elements. If present on any of these input types and elements, the :required pseudo class will match. If the attribute is not included, the :optional pseudo class will match.

该属性不受 range 和 color 支持或不相关，因为两者都有默认值。hidden 也不支持它，因为不能期望用户填写隐藏的表单。任何按钮类型（包括 image）也不支持它。

¥The attribute is not supported or relevant to range and color, as both have default values. It is also not supported on hidden as it can not be expected that a user to fill out a form that is hidden. Nor is it supported on any of the button types, including image.

注意 color 和 range 不支持 required，因为两者始终都有一个值。类型 color 默认为 #000000。range 的默认值是 min 和 max 之间的中点 - 如果未声明，则 min 和 max 在大多数浏览器中分别默认为 0 和 100。

¥Note color and range don't support required as both always have a value. Type color defaults to #000000. The default for range is the midpoint between min and max — with min and max defaulting to 0 and 100 respectively in most browsers if not declared.

对于同名的 radio 按钮组，如果该组中的单个单选按钮具有 required 属性，则必须选中该组中的单选按钮，尽管它不必是该属性所在的单选按钮 应用。为了改进代码维护，建议在组中的每个同名单选按钮中包含 required 属性，或者不包含 required 属性。

¥In the case of a same named group of radio buttons, if a single radio button in the group has the required attribute, a radio button in that group must be checked, although it doesn't have to be the one on which the attribute is applied. To improve code maintenance, it is recommended to either include the required attribute in every same-named radio button in the group, or else in none.

对于 checkbox 输入类型的相同命名组，仅需要具有 required 属性的复选框。

¥In the case of a same named group of checkbox input types, only the checkboxes with the required attribute are required.

注意：设置 aria-required="true" 告诉屏幕阅读器某个元素（任何元素）是必需的，但与该元素的可选性无关。

¥Note: Setting aria-required="true" tells a screen reader that an element (any element) is required, but has no bearing on the optionality of the element.

<form>
  <div class="group">
    <input type="text" />
    <label>Normal</label>
  </div>
  <div class="group">
    <input type="text" required />
    <label>Required</label>
  </div>
  <input type="submit" />
</form>