サンプルです
<button popovertarget="popover">ポップオーバー表示</button>
<div id="popover" popover class="popover-content">
<p>サンプルです</p>
<p><button popovertarget="popover">閉じる</button></p>
</div>
.popover-content:popover-open {
display: flex;
flex-direction: column;
justify-content: center;
align-items: center;
width: 300px;
height: 200px;
border: none;
border-radius: 10px;
box-shadow: 0 0 5px #ccc;
}