网格属性不适用于Fieldset元素

Question

如果将fieldset元素设置为display: grid，则grid-gap将不起作用。

fieldset#grid {
  display: grid;
  grid-gap: 10px;
  box-sizing: border-box;
  border: solid red 1px;
}

.child {
  padding: 15px;
  background: blue;
}

<fieldset id='grid'>
  <div class='child'>
    <div>gc1</div>
    <div>gc2</div>
  </div>
  <div class='child'>c2</div>
  <div class='child'>c3</div>
  <div class='child'>c4</div>
  <div class='child'>c5</div>
</fieldset>

div#grid {
  display: grid;
  grid-gap: 10px;
  box-sizing: border-box;
  border: solid red 1px;
}

.child {
  padding: 15px;
  background: blue;
}

<div id='grid'>
  <div class='child'>
    <div>gc1</div>
    <div>gc2</div>
  </div>
  <div class='child'>c2</div>
  <div class='child'>c3</div>
  <div class='child'>c4</div>
  <div class='child'>c5</div>
</div>

您可以将fieldset更改为div，一切正常。完全相同的代码。唯一的区别是元素。

为什么会这样？

我该如何解决？