<legend>：字段集标题元素

<fieldset>
  <legend>Choose your favorite monster</legend>

  <input type="radio" id="kraken" name="monster" value="K" />
  <label for="kraken">Kraken</label><br />

  <input type="radio" id="sasquatch" name="monster" value="S" />
  <label for="sasquatch">Sasquatch</label><br />

  <input type="radio" id="mothman" name="monster" value="M" />
  <label for="mothman">Mothman</label>
</fieldset>

legend {
  background-color: #000;
  color: #fff;
  padding: 3px 6px;
}

input {
  margin: 0.4rem;
}