、

对表单里的控件分组，

就是这个分组显示的标题

UA样式表

fieldset {
    display: block;
    margin-inline-start: 2px;
    margin-inline-end: 2px;
    padding-block-start: 0.35em;
    padding-inline-start: 0.75em;
    padding-inline-end: 0.75em;
    padding-block-end: 0.625em;
    min-inline-size: min-content;
    border-width: 2px;
    border-style: groove;
    border-color: threedface;
    border-image: initial;
}
legend {
    display: block;
    padding-inline-start: 2px;
    padding-inline-end: 2px;
    border-width: initial;
    border-style: none;
    border-color: initial;
    border-image: initial;
}

例子

<form action="#" method="GET">   
    <fieldset>
      <legend>选择性别</legend>    
      <input type="radio" name="sex" value='male' id="male_btn" form="dd">
      <label for="male_btn">男</label><br />
      <input type="radio" name="sex" value='female' id="female_btn" form="dd">
      <label for="female_btn">女</label>   
    </fieldset>
    <input type="submit" form="dd"/>
</form>

输出结果