Answer by lubar for Which font for CSS "x" close button?

This works nicely for me:

<style>
a.closeX {
    position: absolute;
    right: 0px; top: 0px; width:20px;
    background-color: #FFF; color: black;
    margin-top:-15px; margin-right:-15px; border-radius: 20px;
    padding-left: 3px; padding-top: 1px;
    cursor:pointer; z-index: -1;
    font-size:16px; font-weight:bold;
}
</style>
<div id="content">
    <a class="closeX" id="closeX" onclick='$("#content").hide();'>&#10006;</a>
    Click "X" to close this box
</div>