试图让一个简单的弹出窗口出现在
mouseoverdiv<!DOCTYPE html>
<head>
<style>
.parent .popup {
display: none;
}
.parent:hover .popup {
display: block;
}
</style>
</head>
<body>
<script type="text/javascript">
var e = document.getElementById('#parent');
e.onmouseover = function() {
document.getElementById('popup').style.display = 'block';
}
e.onmouseout = function() {
document.getElementById('popup').style.display = 'none';
}
</script>
<div id="parent">
This is the main container.
<div id="popup" style="display: none">some text here</div>
</div>
</body>
</html>
有几个问题。您在 CSS 中引用类（. 是类，# 是 id）其次，您不需要重载 CSS display none 样式。最后，在这种情况下你不需要 JavaScript。
查看工作示例。
<!DOCTYPE html>
<head>
<style>
#parent #popup {
display: none;
}
#parent:hover #popup {
display: block;
}
</style>
</head>
<body>
<div id="parent">
This is the main container.
<div id="popup">some text here</div>
</div>
</body>
</html>如果弹出窗口在外部（不在父级内部），则：
#popup {
display: none;
}
#sibling:hover+#popup {
display: block;
}<div id="sibling">
This is the main container.
</div>
<div id="popup">some text here</div>只需在 html 标签内添加
title="Some popup text"<button class="toolbar-item-fontcolour"
title="Font colour"
id="btnFontColour"
@onclick="ShowColourSelectorFont">A</button>